How big is 10 caliber?

Properly written as .10 caliber (notice the decimal ?) a .10 caliber firearm would fire a VERY tiny bullet- 1/10th of an inch in diameter- less than half the size of a .22 caliber. However, since very few firms have ever made a cartridge that tiny, you MAY be referring to a 10 GAUGE shotgun- which is VERY large- about 3/4ths of an inch in diameter. With shotguns, the smaller the gauge, the bigger the shell.