I just bought a 2016 CF MOT 400. I have a winch, jut have some questions to help finish the install. Can anyone help me out with what size the bolts need to be for the fairing mount? Also it needs to have a wire to go to the ignition, which wire is the best to hook into. Thank you in advance!

Should be a nut and bolt. I think I used 8mm last time I did one, but just use the biggest that will fit. Use hardened bolts, grade 8, so they don't break easy.

I just measured my 2021 400 to fit a Harbor Freight winch on it.
The winch mount on the ATV has 4 holes with centers making a rectangle 9-3/8" wide by 2-3/16" high.
The winch holes are smaller than 7/16" diameter, bigger than 27/64" diameter, therefore close to 11mm diameter, not threaded.
The fairlead mount has 2 holes about 4-15/16" apart, close to 125mm center to center.
Those holes have threaded inserts that fit standard M10 bolts.

The Harbor Freight 2500 pound winch has only 2 holes 2-15/16" apart, vertically near the centerline of the drum.
The HF fairlead holes are closer together than the CF Moto mount, maybe they are 120mm apart.
I think I can cut slots from the edge of the HF fairlead plate to its holes and make a custom washer to hold it.
I bought 1-1/2 by 3/16 angle-iron. I'm planning to crop 2 angles to fit into the 5-3/4" gap between the CF mounts.
I'll post again when I know how it works.

I put my winch on.  It looks a lot like the factory winch on a 5 or 600 I saw on youtube.
If you see these pics (downsized a little bit for frugality) the 1st one is the parts on a tarp.
From bottom up in the 1st pic:
1. Fairlead holes have been slotted to the edges with a bias to the top so it will fit under a lip on the CF400's mount.
    A pair of rectangular 'dog' washers have a short leg each so they can straddle the edge of the fairlead's mounting plate.
2. The bottom bracket is the one the washers were cut from.  I had to round-file some holes into slots so they would be centered on my template/pattern.
3. The HF / Badlands winch holes were smaller than 3/8" so I drilled them out.  M8 or 5/16" would have fit clear.
4. The top bracket has more metal and the top of the drum lines up with the fairlead so it can use it.
2nd pic: Fairlead installed without pulling the plastic.
3rd pic: Turned out that both top & bottom legs on the angle iron brackets needed to face forward.  That made the locations of the winch mounting holes critical because it had to fit between the upper & lower legs with everything tight.  It did.
4th pic: This is what the CFMoto winch mount looks like through the fairlead mount area.
5th pic: The winch is mounted, plastics removed.  I also lowered the front of the bottom skid plastic temporarily to get wrench room.
6th pic: Plastic is back on and it's waiting for wiring.  I think I'll make a plastic cover/protector out of a plastic coffee can and duct tape, but that's later.
Now to search for wiring guides.  It's remote controlled by a fob so it should be simpler, but I'd like to do a quality job on the power.

Nice work on the adaptor, looks factory.

I wish it wasn't off balance but pics in the 500 manual look a lot like this setup.
Looked at the wiring & remote, super easy.
I only have to find a place to mount the box and wire-tie the 2 cables to the frame.
I think I want a cut-off switch, though, between the battery & the breaker.
The good thing is there aren't any connections to the ATV wiring so I don't have to worry that it might be canbus or tapping into the wrong circuit, or mounting a switch on the dash or handlebar.

I'm moving slow on this, I know, but after putting a new floor in my trailer I wired the winch temporarily to a battery on the front rack using minimum nylon cable ties.  The short leads are on the winch side of the box so the box will almost certainly end up in the front wheel well.   It easily pulled the CF400 onto the tilted trailer and the fob control worked perfectly (except I'll have to swap the motor leads to un-swap the IN-OUT buttons).  The older portable-ish 5000 pound winch I was using on the trailer used a wired IN-OUT pendant that took some planning and added excitement if I tripped on it or accidentally pulled the plug out.
During the test install I didn't check the box for Motor - Battery markings (I assumed the long leads would go to the winch... wrong) and when I touched the battery with the motor-side leads I got the flash of a short circuit.  It was brief and didn't hurt anything but you'll want to pay attention to the markings on the box then connect the motor properly the first time.

The bad thing that happened, though, is I found the right front amber reflector plastic on the ground.  I doesn't snap in, or doesn't snap in anymore, so I expect to super glue it back on its hardware.

Installed the winch harness etc.
I swapped the red & black leads, noticing that one winch lug has a red boss around it, so I could have put the red wire there in the first place.  I cable tied the control box to the structure that holds the fairlead.  I tried to set the box so the wires would not guide water into the box.  Then I fed the 2 wires back and cable tied them.

This is the remote fob using its built-in clip to attach to the standard key.
(I put a little red on the key in case internet wizards could make a copy from a picture)
The manual is there just because I was looking for instructions on the plastics.
The blue shape in the background is a garbage can lid, nothing cooler than that.

Sorry, a bit more, the winch came with boots to cover the wire connectors on the motor but I forgot to put them on.
I will do it soon, though.
And it's nice that the ATV can now winch itself onto the trailer, should be less stuff to pack next time.

I put the boots on a while back, here's a new cropped-from-old pic I might have posted already.
Now I should mention that the wireless control module always listens for a remote fob connection. 
Hence the vehicle battery will go dead if you leave it as-is long enough after you park it.
I discovered that when spring came around.
My solution is to keep it on a solar trickle charge between active times.
An alternative might be a disconnect switch that might also prevent a stranger's fob from releasing the winch.
I expect today's wifi-type remotes make that rare or impossible but it is an HF product after all.
The winch has worked well a few times unloading and loading itself on the trailer.
I think I'll give it a workout soon when the warranty times out and the break-in requirement becomes less of an issue.
(I've had a stay-at-home summer for family reasons).