How much Gigabyte can I place in the Ethernet Shield
I assume you are actually asking "What is the largest SD card I can place in the SD card socket on the Ethernet Shield?"
I believe that it depends on which SD library you are using. With the good libraries it's 32GB (IIRC), and the "poor" ones (ones bundled with ancient versions of the IDE) it's 2GB maybe?
So you should be good with up to 32GB...