The size of pixels you can have totally depends on the power of your PC. If you have a lot of RAM and a good graphics card you can go nuts. I'll tell you from my own experience my machine was not up to much. I had built this thing originally with FPS gaming in mind as well as a few MMO's however a machine built for that is pretty much pansy compared with what you will need for doing any kind of real artwork. I was totally blown away by how pathetic my PC chugged when I started working on a large map. What you might do is check out some of the maps done by others and find a sizing that looks good for what you want and try that. If it's too slow when you are saving things or making changes then you either need to upgrade or cut it down into chunks.

Of course, the pixel resolution you choose has a big effect on how large you can go. If you just want it for your computer screen you might be totally happy with 100ppi but if you want to print it out all super crispy you better think more around 300ppi. Pixels Per Inch will certain effect the file size of your map and that will of course effect the speed of your machine as you are working on it. I can tell you from experience that it really blows to be in full artistic flow and suddenly get stymied by a chugging machine that totally kills off your momentum.

You might take a look at the maps done by a few of the expert map makers here and then try a size comparable with something you like and see how it goes.