tires do come in sizes, but not nearly as many as tire sizes. one tube might fit 700x19 to 25 tires, and another 700x25 to 32 or something. there are two different valve types, 'presta' and 'shraeder', make sure you get the right type tube to match your wheels (the rim holes are different sizes for the valve types).

and tubes are pretty cheap, like usually $10-$20.