Is there a legit way to zoom out more?

There is no legal way to zoom out more. However, if you are not currently playing on a widescreen monitor, and if you can acquire one, picking a widescreen resolution will display a small bit more.

After a bit of poking around, I found some comparison shots on the Team Liquid forums displaying the differences between the three aspect ratios. Each of the pictures below is centered on the Command Center (and I'm not sure why it's German, but it is).

16x9

16x10

4x3

This isn't possible. If the game were designed so that you could zoom out as far as you wanted, the optimal view would be to see the whole map at once. Then you'd end up with competitive advantages for the player with a powerful enough machine to render it all, or players reducing the graphics setting to the point where all the painstakingly-modeled units are mere dots and blobs. The lower limit of zoom level had to be set to something.

Learn to keep a third eye on the minimap! (Then tell me how, because I sure can't.)

Custom maps can lift this restriction and allow whatever camera positions they want, of course.