Our rats wont use their ball

Question
We're the proud new parents of two female rats.  But we're having difficulty with getting them to use their ball to roam around in.  Do rats need to be trained to use the ball?  What is a good size of ball (and wheel for that matter) for rats?  Are rats are both small in size, about 4 to 5 inches from nose to the beginning of their tail.  Thanks!

Answer
Running balls, while marketed for mice and rats, should NEVER be used. There is little to no circulation in them and they are terrifying to them- not fun. You can let your rats run around on top of their cage, on top of your bed, or inside a rat proofed room safely. My rats have a large playtop that hangs over the two largest cages and a tube going from one cage to another and lots of toys. This is how they get their playtime outside of their cages.

The wheels we use are 12" mesh wheels...Any smaller and it would be uncomfortable for them to run on.