If your just starting out and want to try sponge balls, here are a couple of tips.

1. Make sure they're SUPERSOFT

2. DO NOT BUY RED ONES -A lot of venues are dimly lit with red patterned carpets. If you or a spectator drops a ball and it rolls under a table you'll never find it. And you sure as edited can't start crawling under tables. Buy bright yellow or similar

3. DO NOT BUY RED ONES - Every magician has red sponges - be different or buy Earplugs

4. If your new to sponges buy the 1" or 11/2" to start with until you get confident with them

5. ***MEGA SPONGEBALL TIP***
Run the spongeball under the warm water tap (fawcett) until the ball has soaked up as much water as possible. Squeeze the water out and towel dry. The ball will feel slightly damp but it will be at least 1/3 bigger in size and be super springy keep it in a poly bag until ready to use.

I do agree with Merlinuk's opinions of Supersoft and using warm water to restore size and spring to your sponges. As far as color, use a few different colors, and see which works best for you. Most magicians do use red ones (I do), so if you want to set yourself apart slightly, use a different color, or perhaps you can come up with a good patter or routine that involves mention of the color, a color change, etc. Size will depend on your hand size, and your confidence. I use 2 inch Red Supersoft, and they suit me just fine. And the balls do compress and get a bit dull after time. Using warm water, even a little bit of soap if you're so inclined, will help revitalize your sponges.