Depends on Age sometimes 11 inch or 12 inch balls
High school softballs ar called 14's. They are the regulation size of ball for any level of competition above 14 and under. You can buy them at any sporting goods store, i would suggest the leather balls, they last longer and you can get a better grip in them.
You can play softball as long as you like just not a grandma. There is pro softball, so you can play softball as long as you like.