There appears to be no published SD card size limit, but in this forum and other forums, users report that SD cards are limited to 32GB.
This may be of interest:
Expand your iPad's storage capacity with HyperDrive
On the road with a camera, an iPad, and a Hyperdrive
SDHC 32 gig cards should work, the problem with SDXC cards is that I think that they are ExFAT pre-formatted, which the iPad and the kit doesn't recognise. This page lists the supported transfer types (for direct transfer from a camera) and card types :
- Picture Transfer Protocol (PTP)
- Mass Storage Device protocol
- SDXC (supported only when not formatted as ExFAT)