I find the same thing; in Plesk 6, I can't set a limit bigger than 2047MB. 2048 won't work. I asked over at the Plesk forum, got no real response.
The domain I'm trying it on wasn't made with a template; perhaps you can make a template that's bigger than 2047MB and make a domain from that and then it would work. I haven't really messed with it, "du -h" is good enough for the time being.