I always over-buy my skimmers so you don't have to upgrade down the road.
30" is a good amount of room, but you will be limited by your sump size.
What is the footprint of the section of your sump you can put your skimmer in? I would buy the biggest skimmer that would fit into your sump.
By footprint I mean like 12" by 8" or whatever.
You can see the footprints of ASM skimmer here:
www.asmskimmer.com