I use a Sandisk Sansa Fuze v2 with a 64GB micro sdxc. It was a trick to format, but it works fine now. I'd assume Rockbox doesn't limit the card capacity, as long as you can format the card to FAT32.
You could always buy an older player (like the iRiver H340 or Toshiba Gigabeat F) and replace its HDD with a higher capacity drive. If you don't mind messing with the iPod video, there is a 240GB HDD available for it. Just install Rockbox and thank Apple for the hardware while giving iTunes a kick in the backside.
As for standard SD cards, the full size SDXC goes up to 128GB, but I'm not sure which stable Rockbox targets use regular SD cards.