Can I change chainring size on a dirt jump bike? I don't know the name of it but there is a part on single speed bikes. The gear? I think right where the crank is connected. Well on bmx bikes and especially dirt jumping bikes it is really small and if it is small then that means you have to pedal a lot more to go faster. Well can that gear part or whatever is connected to the crank be changed to a bigger single speed to pedal less? Sorry for not knowing the name guys please help though it's important before I buy this bike.
It's called a chain ring and, yes, it can be changed to whatever you want it.

http://www.commencal-store.it/shop/895-thickbox_default/mountain-bike-dirt-jump-commencal-absolut-al-2014.jpg

The dirt bike above has cranks which seem to use chainrings of type 104mm bcd 4-bolt. So any 104 bcd chainring will do the job. E.g:


(source: cyclesportsuk.co.uk)  


  if it is small then that means you have to pedal a lot more to go faster


That is correct, but usually those bikes have a very small cog at the rear. So you really don't have to pedal like crazy to go fast. Note though that the main purpose of those bikes is not to go fast but to jump.