Is it important how big the tympanostomy tubes are?

Yes, in length: In general, the longer the tube used, the longer they will stay in before naturally working themselves out, and the longer the child benefits from them.

Depends on kid: Short barbell like tubes the size of a bb shot are often the only tube a kid would ever need. Some kids with a long term need for tubes will extrude the regular ones quicky & a longer t-tube style will tend to stay longer. The outside diameter & inside window of the tube are limited by how big a hole you can create in the ear drum & expect it to heal over after tube is gone.

No: They are usually standardized for the size of the child.