Most sheep vaccines are 1 or 2ml. Mine is a 2ml injector, just be aware don’t buy anything with sterimatic system as it’s not being manufactured anymore (there maybe some residual old stock guns in the shops still).
As for drench guns, mine is 10ml- I get on fine with it, if I need to do any more than 10ml I just do 2 pumps (fluke drenches are normally 2ml per 10kg so 14-16ml per ewe, I do 2x 7 or 8ml pumps).