^^^^ Sad thing is you can do a guide but sizing can vary each season.
I've had a few pairs of FBTs over the past few years and each have been different in sizing. My size 11 neons fitted small, my waxed two tones in a 12 fitted perfect at first with no possibility of sizing down (they stretched a bit and are a little on the roomy size now but totally wearable) and recently I picked up the FBT classics in an 11 which fitted slightly small at first but now are straight up perfect.
Likewise with Logans, mids in an 11 were a tad small but wearable, lattice in a 12 are great and lo's (original fake version) I tried on in a 12 are fine but a little tight although I think after a few wears they'd have been wide enough.
If possible, try on all Visvim shoes if you have the luxury of doing so.
BTW - does anyone have a pic of someone wearing a 20L and 22L Ballistic? Need to know if it looks normal on someone who's 6ft+. I've seen a pic of an asian bloke with it on, guessing he was under 6ft and it looked normal.
Going to buy the new Ballistic 20L Hexagon with Kangaroo-hide bottom :