Hello,
I am planning to get my first fat bike - possibly a Salsa Mukluk. I am between a small and medium?
My height is 65.5 inches and my armspan is 64 inches.
I test drove both on paved road and the medium felt slightly better.
I plan to use it for winter commuting on snowy trails and road.
Any advice is appreciated.
Thank you

Welcome to the forum. Always a tough choice and a very personal one. I am between a medium and large on many sizing guides. Have always gone for the larger frame which has worked out so for. Best of luck with your choice.

Thank you very much!

If a frame is slightly too small you can get seat posts and bar stems for a better fit. If a frame is too big there is nothing you can do to make a better fit.

When you say you are between sizes, is that according to your height, inseam, and Salsa’s sizing guide?

Or do you already know the geometry/fit numbers you’re looking for, and you land between sizes?

If the former, go with what felt better.

it is also worth looking at the stock stem lights. This can give you an indication whether you can go longer on the medium frame or shorter on the large to fine tune fit.

Don’t mess with a set backseat post solely to deal with a frame that’s too short. Seat-post setback should be largely independent of frame size, unless they seat tube angle is different between sizes.

So there is a lot of factors involved but will say this one up front, you can make a smaller bike bigger (longer stem, seat set back, offset seat post, wider bars...) however if your frame is too big you are very limited and kind of in a bad place.
Now I am not going to tell you which way to go, everyone has different opinions and what they prefer. Myself..I prefer a smaller frame than a larger one. I fall in between a Med and Large, I go with Med, I find it is easier to throw around, handle, this is for both Mountain, gravel, CX and Road bikes, I just picked up a Farley 9.6 last week (why I am on here looking for ways to slim her down more), and I rode Salsa's and Treks both back to back, I am a bit bias with Trek (own many Treks) but I really liked the colors, options, quality, cable routing of the Salsa, however the Farley felt very similar to my Trek Top Fuel 9.8 fit/feel/groupset..
Saying you 65" tall (5' 4" ) puts you on the shorter side, and would put you in a small frame, unless I read your size incorrectly, I would go back and give that small another try?

I tend to size down in marginal situations, but that's mainly because my inseam is a bit short for my height.

I'm a fan of sizing up...a bit easier to make a slightly bigger frame small...than a small frame big

If you're going really fast go bigger frame
If you're going slow, go smaller frame
and since it's a fat bike.....

One important factor for going smaller ( my preference generally and I'm a 5'10" guy) is that a smaller frame is always appreciated when your boots sink into soft snow---your crotch won't like a frame that juuuuuuust fits you on hard pavement.

Just today I was riding with a woman friend whose fatbike really should be a small, is a medium, and when you sink in snow or stall out on a climb due to lack of traction, one appreciates a smaller frame.
Today we were on some really hard trails and I sure as hell wouldn't want a larger frame, having to suddenly jump off my bike, stalled on some very steep and narrow trails, with a steep dropoff to one side, which happened to me and I was getting tired too.

My wife is 5ft 1 or less and her small fatbike frame is sized ok, but close. We just couldn't find an XS like her touring bike, which fits really well.
Don't forget though, different frames have different frame shapes, so toptube positions do vary.