It varied, but, as a general rule, things were cramped. You might try looking at plans or research materials on a specific ship. several, including the CSS Missouri and CSS Atlanta, were surveyed and the information is available. Her is an excerpt of the survey from the Missouri: Length over all, 183 feet; breadth of beam, 53 feet 8 inches; depth of hold, 10 feet 3 inches; length of casemate at base, 130 feet 6 inches; length of casemate at top, 105 feet; width of casemate at base, 53 feet 8 inches; width of casemate at top, 29 feet; height of casemate from water's edge, 11 feet 6 inches; draft of ship (reported), 8 feet 6 inches.