You cannot specify this. You can only limit the size of the cache folder (SharedUpdates) in the LiveUpdate policy.
If you have an environment with 32-bit and 64-bit OS, calculate 2 x 550 MB, plus size for delta files. The delta files are in most cases very small (100-500KB), however if a client was off for a longer period, it will get bigger delta files. So I think 2 GBs for the cache folder are sufficient.
Symantec's old default value of 500 MB is ridiculous because it's even smaller than one full content version.
Please read this enlightening thread (in particular SMLatCST's posts):
https://www-secure.symantec.com/connect/forums/gup-space
Operating System: Windows Server, running 24/7
Processor: Every modern processor should do it.
RAM: 2 GB or more