When I carried at 3-4 I went up one size. When I started to carry appendix I went back to my normal size.
Single stack or double stack makes no difference. The grip is not down inside your pants
I usually wear the ones with flex material, either jeans or business casual. I find these are more comfortable when not carrying and work good when carrying. I also use a good belt and sometimes a pair of undergarment suspenders, they help with the extra weight on the belt.
Same size pants if you wear a relaxed fit pants, up one size if you wear a tighter fit pants. Slim guns like a P365, Shield ect might shift things down a size relative to a full width double stack gun also.
ATM I am carrying a Glock 19 and mag carrier IWB inside a +1 sized pants. I am wearing a +1 sized pants mostly because I have lost enough weight to drop a size but haven't bought new pants yet. I have also worn this same setup down a size and it still works, just a bit tighter. Flex fit pants are handy for this.