Well, I found something on a site that tried to give me an idea thru part numbers of what was compatible, and it seemed that 64 was okay. Then I saw in Kingston Memory's site that they sell 128GB cards that they claim work with them.
I ended up with 2 64GB cards, on a Samsung and one a PNY Elite. Put the PNY in and it was recognized and works, so I didn't even try the Samsung. the PNY tells me I can do something like 2800 RAW+JPEG on the 580, so I should be set!
A couple of a350's, an a700 and now an a580, plus even more lenses.