Many manufacturers specify "max 32Gb" or something to that effect. That is because they are careful and don't want to claim compatible with 64Gb while they have not been able to test them. So at the time of manufacture, the biggest they could get to test with was 32Gb, so they say: compatible with 32Gb, but there is no real reason to suspect that 64 or 128Gb won't work.
When passing the 4G limit, the SD cards suddenly became SDHC cards. /that/ required some firmware/software update because there was a 32-bit addressing limit in SD(-not-HC) cards.
As far as I know, no such limit is "in sight", so the biggest you can use is the largest you can find.
