Bottom Bracket and other ideas.
 

I'm fairly new to biking. So, I was hoping someone could explain how the bottom bracket size is measured, like if I wanted to buy a new one. Is there a special bottom bracket for fixies or can use any one?
Secondly, is it possible to buy one of the internaly geared rear hubs and put on a fixed cog and then be running a three speed or whatever fixed gear?

The answers to your questions.

http://www.sheldonbrown.com/gloss_bo-z.html#bottom
and
http://sheldonbrown.com/awfixed.html

So are you looking to replace the bb? Or are you getting a new crankset? Tell us that first. And also what bike it is.

Neither, I'm just curious.