Lucien,

>Is size the only limitation with MS SQL? Is there a limit on the number of users?

1 CPU (multiple cores/threads)
1 GB Ram utilized
10 GB per DB
needs windoze server
T-SQL closer to vfp syntax, Advantage uses similar syntax if linux install becomes necessary
used to be cheaper for small installs needing payed for servers (HAVE NOT RE-ANALYZED LAST YEARS)
each install easy and point-click interfaces easiest (but with annoying changes between versions - MS typical)
great official docs
On UT more brains to pick

DB2:
2 CPU
4 GB RAM
no size limits
(support CAN be bought)
Oracle syntax can be utilzed as well as IBM dialect
painless install by now
used to be between MS and Oracle prices (HAVE NOT RE-ANALYZED LAST YEARS) if payed install needed
great official docs

PostgreSQL
no limits CPU
no limits RAM
no limits DB
some support via 3rd party can be bought / no big company behind it
nowaday painless install (was PITA early this century!) even on windows if one needs it
docs good but not clearly up to other 2 level which need to please paying customers
help via own usegroups - great brains, but different tone.
Behind many other dev fwks now as default install (Servoy, xDev) as it is free - it is not a homegrown buggy OS thing.

Oracle / (owns MySQL as well nowadays):
won't discuss, personal dislike - but works and some big names use it ;-)

SWAG:
if CPU limits usage with todays HW, you have so much traffic that you should let some users pay ;-)
10 GB is an annoy/PITA factor aimed at trapping you, but you could use multiple DB.
1GB gets stretched across DB / index sizes and number of users.

from pure biz (discounting vfp-SQL knowledge, MS-server experience) POV PostgreSQL or DB2 cannot be topped IMO. Deciding which to pick is a question between troubles to get it allowed on the client server and the tone/level of support you want/need.

If you have a specific client in mind, his wishes are the law - pass the buck.

vfp-SQL and T-SQL similarity:
gets you clearly up to speed quicker. But in todays world windoze grows into a liability (personal opinion!)
even if your client group currently wants only that - even in case of the 1 huge client only ;-)

NB: All of the above from vfp upsizing post-2000.
Nowadays I am looking closely at Lianja - for usage on Desktop, Web, xPad and xPhone.
Lianja also has a "real" SQL backend offering. Dunno if you have similar needs/plans, and it is quite new, even if re-using solid and debugged code base. Not if you want to start coding next week, but if your plans are more long range, I'd reccommend checking them out as well - perhaps you might change your itinary. WinDev offers similar options. Don't want to intrude, as you asked a specific question ;-)

my 0.02€

thomas