Not a stupid answer but a very complicated one. There was a great variety of weapons used in the CW by both sides and books have been written on the subject. To make a long and complicated answer short, the main rifled musket ammunition used by both sides was .58 cal. for US Springfield made weapons and .577 for the Enfield Rifled musket. Bullets were eventually made sized to fit both to ease the distribution of ammunition.

The second most common caliber of rifled musket for the CS forces was the .54 cal. There were many imported Austrian Lorenz rifles and the home grown M-1841 Mississippi rifle that were either.54 and .58 cal.

Smooth bore muskets were usually .69 cal.

The most common size for pistols was .44 cal and .36 cal. There were many calibers and types of ammunition for the variety of hand guns used.

I would suggest you read Dean Thomas book, "Round Ball to Rim fire", Vol 1 for a adequate understanding of CW small arms ammunition.