I was thinking of using a 30man swordmen block with handgunner detachment in a 2k list, ´what is a reasonable size for the handgunners?
None! :P
Id use archers as a missile weapon detachment, because they are best at screening, can move and shoot and are hard to hit.
If you do use handgunners, Id go for ten. That allows them to take two casualties without panicking, they are enough to cause some damage and they can gain a rank if it should be necessary; at the same time they are not too expensive - an important aspect for vulnerable detachments.
as I'm going with a defensive strat they don't need to move that much, so the extra punch is welcome...