Recommended partition size of Windows 2003 Server Standard Edition

for the system partition i take at least 3 gb
put you logs on other partitions

If you set things up properly (logs on other partitions, data files (Exchange) on other partitions, then I'd probably make the C: drive 5-10 GB.  You don't want to underestimate because future upgrades can become PAINFUL.  But you don't want to over estimate either because the space could be used for file sharing, etc.  Basically, only you can answer this correctly.  What else do you plan on or think you might have the remotest need of installing on the system?  Allow space for those items now (plus at least 1GB extra for upgrades).

By the way, Windows won't install if you don't have 1.5x RAM in free disk space - so if you plan on ever upgrading, RAM, make sure you have 1.5GBxRAM + sufficient space for Windows and whatever apps/server programs you may use (You can always get around the 1.5xRAM thing by removing RAM to do the install and putting it back, but Windows will typically create a page file during the install and use 1.5xRAM.  You can move the page file as soon as Windows is setup, but until then, you're stuck with it on C:

keep in mind that you can move the page file to any local drive... to it doesn't have to be on the system/boot partition at all.

you better never do that ,
place it on another physical drive   !!!

if possible of course

its on the boot partition by default