Shop vacs aren't all they're cracked up to be. These days a lot of regular home vacs or even hand vacs suck as hard as shop vacs. Shop vacs can suck a lot of air in a hurry, but they don't really suck harder than a good canister vacuum. They're also big and bulky.
The gallons don't matter; if anything, smaller is better; the vac sucks air through the big bucket, so the bucket just holds more air to suck out. (Probably true for hoses, too. A 1 1/4" hose is sufficient for at least a 2 x 2 foot table, and the 2 1/2" hose just holds unnecessary air. It probably doesn't matter much, but the big hoses tend to be awkward.)
If you have a shop vac sitting around anyway, by all means use it. But if you're buying a vac, I'd recommend going to thrift stores and getting the one that sucks hardest for a lot less money.
One way to see which sucks hardest is to put a piece of craft foam over the end of the hose, and see how far into the hose opening it dimples. Whichever vac pulls it furthest in, buy that.
For my small (12" x 18") platen, I use a Euro Shark Ultra Big hand vacuum, which is actually quite small. It is about a tenth the size of my shop vac and actually sucks a little bit harder. I'm not sure it sucks enough cubic feet per minute for a big platen, but it's plenty for the small one. It cost me something like three dollars.
There are no good specs on vacuum cleaners. What you mainly want to know is how hard a vacuum they can pull, not how many cubic feet of air they can move.
Horsepower isn't a reliable indicator of how hard a vac can pull, but on average more is better. How hard it actually pulls depends on the impeller design, and whether it's designed to move a whole lot of air against low resistance, or less air against a higher resistance. (Basically a torque-like "gear ratio" tradeoff.)
I wouldn't look at horsepower, particularly. I'd look for amps or watts. (At a given voltage, like regular wall outlet voltage, amps and watts are perfectly correlated.) That will let you compare across shop vacs and regular vacs.
I got my little hand vac because it's got an 8-amp (1000 watt) motor in it. (Most old hand vacs are only 2-amp; newer Dirt Devils are mostly 6.5. Unfortunately Shark doesn't seem to make 1000-watters anymore; they top out at 700.) Once I got a vacuum gauge, that turned out to be a good call. The little sucker really sucks.
Another possibility is an electric leaf-blower type "blower vac." That's a lot like shop vacs without the big worse-than-useless bucket.
But if you really want a shop vac, I have one to sell, which I bought before I realized these things. Hardly used it at all...
