Well, V2 Dats are quite very very small and a free disk space of 50MB??(perhaps) would suffice.
Unless you're updating manually using an SDAT. I believe you need at least 150MB of free space for the SDAT.
Assuming VSE 8.5/8.7:
I don't know the exact answer, but the amount of disk space required will be small. I'd estimate *at the most* - 250-300Mb.
If a machine doesn't have that much disk space free, then it already has big issues.
There are some bugs in various McAfee products where old dat files aren't correctly deleted from the products, but even then - I don't that sort of issue would be relevant.
The basic update process for VSE 8.5/8.7:
Download several files from update repository (approx 500Kb max, unless dats are very old - where it will download up to around 7Mb (going back to around 5 weeks worth of dats), or if older - then ~60Mb for the full dat file zip).
The various dat upgrade mechanisms would require up to ~200Mb free on the machine (they need to "patch" the dat files with the changes, then back them up).
But overall, A machine with less than that sort of disk space free already has big issues.