The 1TB one is due to the way Clonezilla images work, it expects a drive with at least 1TB capacity to mirror the image back.
Re:SteamOS: It's here
by binaryatrocity on Saturday December 14, 2013 @12:06AM
(#6904713)
The 1TB requirement is for the default install, if you use the custom Debian-based installer that isn't an issue.
"The image provided here requires at least a 1TB disk."