How to minimize Windows Server 2008 R2 size?

These days Microsoft is recommending that C:\ be no smaller than 32GB for R2 installs (the official word). Microsoft has gone to some lengths make installing new features Just Work without having to dig out install media. What's more, it keeps complete 'undo' history for system files, and Microsoft seems unimpressed by those of us who want to keep C:\Windows to be under 20GB in size. This is a far, far cry from when you could install Server 2003 on a 10GB drive. They really don't want us going back to that.

I recommend, if you can, installing Server Core to give you the smallest possible install of Windows Server 2008.

There are other things you can do with your core install to further reduce it's footprint.

This guide will help you also understand what server core is.

Edited after you've updated your original question:

You didn't tell us how big your SSD drive is. I'm going to assume it's not sufficient for your needs. You also didn't tell us what the machine was going to be used for. You asked about SERVER software so I think we assumed you were installing a server.

Hard drive space is relatively inexpensive these days. Ditch the SSD drive and go with a ginormous conventional drive, install Windows 7 with VS2010 for development and then install Sharepoint in a VM on the same machine. If you can, I would also bump up the ram in this machine.

A "server" machine, on a SSD drive? weird decision, imho. What will be the main role of this server?

Maybe you'd be better off with Windows 7 but it's not like it would really make the footprint smaller as you still need the x64 version (which has a bigger footprint) - and Visual Studio is a beast, you'll want at least 80 GB of space for that system and it will start to be annoying after a few months of updates... 160 GB would be my suggestion.

See this document on the options for Sharepoint development. A separate server (physical or virtual) is also an option.