The Eval kit has 1Gb SDRAM and 2Gb (or 4Gb) Nand.
I am trying to figure out what is the largest size SDRAM and NAND Flash that I can connect to the SAM9X25?
How can this be calculated?

Thanks
J

The NAND chips are accessed in a page/block/sector fashion like a hard drive, their physical memory foot print is consequently very small. The size limitation would be the software you have to access the device. The biggest problem is the constant obsolescence of parts, so have a clear strategy for parts/footprints.

I can't speak to the SAM9X25 directly, but most of the 9xxx family have chip select windows of 256MB

If you need a lot of flash, consider using an SD/MMC device. With that 2GB (16Gb) should be no problem.