You really need to talk to a lighting designer or study the websites of the manufacturers well. Depending on the bulb you use, the ceiling height, and the size of the can, you can size and space the system out. Just changing a bulb from a flood to a spot makes a big difference, and changing it to a "standard" lightbulb, radically changes the light level and pattern, too.
The better websites will show you the angle of coverage for their fixtures with various bulbs. Then, based on how high the ceiling is, you can determine how close to put them to create an even coverage. Now, that may not be what you want. FOr example, you might be trying to highlight some artwork, or feature. One manufacturer that I've played with is www.waclighting.com. There are others that make similar such as www.cooperlighting.com (not absolutely sure on this one - they make Halo stuff, if I remember). But, the general shape and bulb characteristics are very similar for each manufactuer.
So, spend some quality time researching, or buy the stuff from a lighting store that offers design help.