One easy way to measure the "size" of a ball is how far it is from side to side. Your tennis ball example is a good one: a typical tennis ball is about 2.5 inches form side to side. And one way to measure that size is to place the ball on a table in the sunlight when the sun is nearly overhead -- close to noon.
The ball's shadow is then almost a circle, and you can measure the width of the circle with a ruler, and that'll be the width of the ball as well.
Another way to measure this is to put the ball between two (large) books and hold the books parallel; it's then easier to measure the distance between the books with a ruler.
Finally, you can wrap a string around the middle of the ball -- the very widest part, like wrapping around the equator of the earth -- and mark the string with a pen so that between the two pen-marks is exactly one trip around the ball's middle. Suppose that this comes out to be 42 inches (which might happen for a kid's kickball, for instance). If you divide the length (42 inches) by the number 3.14, you get
42 / 3.14 = 13.38, more or less
and that tells you that the ball is a little more than 13 inches across.
The number 3.14 is special -- it works no matter how large your ball is: you divide the "length around the ball" by 3.14 and you get the length across the ball. (The actual number is a tiny bit bigger than 3.14, but the difference only matters when you'll doing very precise things; 3.14 works for almost all practical purposes.)
I hope this helps a bit.