just take it out and take it with you to an auto parts. Most people who work at auto parts stores will know the size you need. When i went auto zone to buy mine i pick up the wrong one. The guy checks his computer database and tells me what i need. just ask someone where you plan on buying it.
I bought mine at costco. Problem is they don't offer an exact replacement for a GenIII so I had to modify the cover and flip the cables. But I got like 40 more cranking amps and they are the same dimensions.
just go to wal mart ... they can match ya up, and dispose of the old one i believe... and on another note, since its winter out, if it gets cold by you very often, a lot of cranking amps can help start a car in the winter!