Diceman: This other thread might be of interest to you if you’re looking for sites that can tell you about specific MP3 players. C|Net also has a nice guide to choosing an MP3 player.
Ultrafilter is right (roughly) about the filesize. As for WMAs, they will be the same size if they’re encoded at the 128 kb/sec bitrate. However, you can get roughly the same quality using 96kbps WMA as 128kbps MP3, so you can fit more WMAs on your player than MP3s. Ultimately, though, it depends on the choices that Napster offers you.
If you want to see a chart by # of songs, just go to the iPod specs page. Apple lists iPod capacity by number of songs each iPod can play, but the number will apply to any other MP3 player of the same size – that is, as long as the songs are still encoded at 128kbps.
Some music providers offer higher bitrates, others offer lower ones. Basically, the higher the bitrate, the higher the quality, but also the more room taken up. There’s always a tradeoff between how many songs you can have, and how good each one sounds. 128kbps is a good balance for most non-audiophile users.
As for hard drives, any MP3 player can break if abused. A large part of it depends on sheer luck, even among different users of the exact same product. Hard drive players aren’t especially fragile, but you wouldn’t want to deliberately shake them in an attempt to break them. Walking, biking, etc. should be fine, but if you’re doing a million nonstop jumping jacks, you’d probably want to get a flash player. As always, your mileage may vary and different people will say different things about semi-stressful activities, such as running.
If you want to be on the absolute safe side, get a flash player. You can shake 'em all you want and nothing will happen, and they usually have longer battery lives (not always). The main disadvantage is that they can’t hold as much as the hard drive players.