Often overlooked, skate hardware is what keeps everything together! With countersunk mounting options for top mounts and ATVs and Panhead for drop and drop throughs you can find the right skate hardware for your board. If you stand over your trucks get counter sunk, if not get pan head.
To generalize, 1.5" is is usually what we use on our longboards and 1.25" is what we use on most cruisers and ATVs. This is roughly based on riser height so if you're using 1/8" risers go for a 1.25" bolt, if you're using 1/4" or 1/2" risers then 1.5" is the way to go.
If you're setting up a complete make sure you've got risers too!