That's a tricky question as it all depends on the individual and their usage in Windows vs OSX. 20GB is enough for Windows 7+all updates and a couple small but essential programs like adobe reader, but just bearly.
Ditto, it just depends all on the usage and the individual. I have a work laptop that all I use it for is my work (Lenovo T61...yuck). After one year, I have 21.5 GB used, and over 90 GB free.
I have my MBA set up with Leopard and Bootcamp. My original Bootcamp partition was 25 GB, and it was plenty, until I started going crazy on Steam and downloading a bunch of games.
If I was disciplined about getting rid of old stuff and organizing, the 128GB SSD would be plenty. I have my old PowerMac G4, and I basically have every photo I ever took (even the ones with the thumbs in them), every song I ever ripped from a CD or recorded using Audio Hijack Pro, every download, etc. for a number of years, and I am probably up to about 250GB.
My "dream" is to take about a week (month?) off and organize my iTunes Library & iPhoto Library, create multiple libraries, and then archive the old stuff and just put the recent/important stuff on my MBA. I think I could get it done on the 128GB SSD, but it really does seem like an excuse to upgrade! And this is with a now 35GB Bootcamp partition, albeit, I have Win XP, not Win 7.