FL tends to fit truer to size and maybe a little bigger than usual. KUIU is opposite, size up on most of their pieces. I think it really depends on the piece of clothing you're looking at.
Attack pants fit like blue jeans for me as per sizing. In jackets the “Guide” was tight as heck in the armpits and chest but Chugach and super down (and spindrift) fit in my normal size. Good luck.... I like all mentioned clothes very well except the Guide jacket which was returned.
I've had to go up one size for all my Kuiu top pieces while the Kuiu bottoms were true to size. Kuiu tops I sized up are 125 and 145 merino tops, kenai puffy and guide jacket. Kuiu bottoms that fit true to sized are 145 zip off bottoms and guide pants.
I have the First Lite corrugate guide pants and they are true to size or just a touch on the large side.
I wear a Large shirt in pretty much everything and FL medium fits me perfect. I’m a 33 waist and FL medium is also perfect. I have a lot of their stuff and the tops are all one size big with one exception... FL Sawtooth I wish I would have got the large.
I'm 5'10 225. I wear XXL in just about every top hunting or regular garments. For me Kuiu merino XXL tops are spot on, perfect length in torso and arms. FL LS merino has been frustrating. I have a chama in XXL and it fits great except for the arm length, it's like several inches too long. I have to roll up the sleeves and the thumbholes are useless since I have to roll the sleeves under. I have a LS wick in XL (I downsized due to the long sleeves in the chama), sleeves on this shirt are still long but its doable, a little snug in the arms and chest. I have a wick in SS in XXL and it's perfect. I have a 28' draw length on a bow so I don't think I have short arms or something. I wear attack pants in 36 and they are perfect. I have a 32' waist and 32' inseam. Never tried FL pants.
I normally wear a XL (in fact I think I own maybe 2 XXL shirts total and zero L ) I got a XXL, and it was just a tad to tight around the chest. I got the XXXL and it fits perfect around the chest but the arms are a tad long. I love the coat but I doubt I'll buy Kuiu in the future because of the sizing just isn't right for me.
6' 195lb, 42 chest, 34 waist and inseam. 34 attacks were ball pinchers so 36 attack pants, Chinook pants are perfect. Medium pants in first lite. Medium tops in everything except Kuiu jackets, those are large. Medium jacket or vest fits but just barely. Guide vest is fine that way.