Get a copy of Sign Structures and Foundations from signs of the times. IIRC they have a newer book with more complicated formulas if that one is out of print. Then make up a spreadsheet to figure it each time. It's not rocket science, even though engineers will act like it is. It's simple algebra to get a "close enough" answer so you can price the job. Now, the figures you come up with will be adequate for estimating but you will still need to get things stamped by an engineer in order to get a permit approved depending on your state regs. I once (in NJ) needed sealed plans for a set of styrofoam letters glued to a wall inside a store. Since it could be seen from outside the store (you had to catch it just from the right angle through the glass) the town insisted we get a permit and sealed plans. Yes, that really happened and was IMO a money grab by the township. Sort of like that same town requiring a ZONING permit for signs inside their mall.
Also, and again IMHO, the reason that most towns now require sealed plans is so they can play CYA. If they didn't approve it, they are not liable. In any case if there is a problem, it's the sign company that will eventually get the blame.
Anyway, based on a 3x5 DF sign, 6' OAH I came up with 0.345 cubic yards (48" deep footing assumed) based on a 115 MHP wind load when using my spreadsheet. Go to up a half a yard to be safe. And make sure you quote it with a disclaimer in case your engineer wants more concrete. Also, a 4" round pipe with a 1/4": wall should be more than enough.