dopefisher11 , it all depends on your player's available space for the replacement battery. You need to measure the maximum length, width & thickness. Some sellers on eBay lists the ipod battery dimensions, but not sure if they are accurate. For my player, only the ipod shuffle fits.
The higher the mAh rating, the longer the battery lifespan, so go for a higher rating as possible. The only thing to look out for is the voltage rating, which is usually 3.7V, not much of a problem.
There is a post
Here on using a nano battery. You might want to read it up.