The size of the hole in the barrel. Since there are high and low grooves cut to spin the bullet, it is the distance between the high places (called the "land" of the rifling). If that distance is 0.030, it is a 30 caliber weapon. Shotguns are done differently. The "gage" of the shotgun (except 410) refers to the number of round lead balls the same size as the hole that it takes to equal a pound. A 12 gage shoots a 11/3 ounce round lead ball and it takes 12 of them to equal a pound. A 16 gage shoots a 1 ounce round ball, etc.
No such thing. You can have small caliber but not low caliber.
Some guns yes, but most guns would require gunsmithing to change caliber.
diameter
Pellet guns tend to come in two standard caliber sizes. The first smaller caliber is the most common for pellets guns and is .177 caliber. The second is .22 caliber that can be found on some higher powered pellet gun models.
7.62 mm (or 30 caliber) and 5.56 mm (or 223 caliber) are common rifle cartridges. Pistols, machine guns, and artillery will of course be different from these.
Length, not caliber, determines action size
The caliber
.68. Almost all guns are this caliber.
The 308 caliber guns are more expensive than 5.56 assault rifles.
44-40 ?
Mostly all of the paintball guns made today are .68 caliber. Though with the recent changes in paintball some guns are being made with .50 caliber. Though by the year of this gun, it is 100% of .68 caliber.
Artillery