mouse needs alot of space

I've just purchased an iMac (desktop computer) that comes with a wireless mouse. I find the mouse needs a lot of space to get the pointer where I need it to go. I've gone into system preferences, and there are things I can change there about the operations of the mouse (e.g., speed, inertia vs no inertia, right click, etc) but I don't see the option of making the mouse more sensitive, such that the pointer can travel across the 21" screen without having to travel about 6" on my desk. Any help would be appreciated.