I think I read somewhere (long time ago) that 8 GB is maximum, people reported they got this working. But the filesystem of the iLiad only shows that 4 GB are available.
Here's an overview of what people reported "from the field":
https://wiki.mobileread.com/wiki/Iliad_Memorycards_list
just noticed that someone reported he got a 32 GB CF card working... That's the theoretical limit for a FAT32 filesystem