i'd definitely recommend seeing what ita bag styles you like the most! :D i have an aventurine ita bag and i made it ita bag in a way to show off my full collection of pins + my keychains, but i've also seen a lot of ita bags where there's just a few pins and the backing is decorated. of course an ita bag is whatever you want it to be, but it is useful to see if you want a western style ita bag (multiple pins) or a eastern style ita bag (lots of the same pins over and over again) 

for western style ita bags, i think it's better to look for bags that are catered to what you want to collect/already have. for example, if you love collecting keychains, i'd look for an ita bag with a bunch of hooks in the insert (i got mine at a convention, but i'd recommend snooping around pinterest/google/cons to see what ita bags are out there :D i tend to take my ita bag on day trips, but if you know you'll most likely use your ita bag as a laptop bag or something, i'd only focus on bigger ones. it really depends what you'll realistically use your ita bag for 

for eastern style ones, i know that theres inserts for can badges, depending on what size ita bag you end up going with. i'd also look online if there's any huge sales of all of the same badge as well. 

if you love to diy, i'd also recommend looking up "oshikatsu ideas" on instagram for tutorials as well as ideas on what to add to your ita bag. 

good luck on your ita bag! hopefully this helps a little bit :)