Some of the most memorable fish are those that barely make it through a big hole. Three of mine come to mind, a lake trout and two pike each of which barely squeezed through 9-inch holes. The lake trout I estimated at just over 30 pounds. One of the pike measured 44 inches and weighed 28 pounds. The other pike hit the ice last season and was one of the biggest ever for television, a fish that was 48 inches and we estimated at over 30 pounds.
I have never had to enlarge a hole to get a fish out, but other staff members have — 50-plus inches of muskie does not fit through an 8-inch hole. Nor does a 40-pound flathead catfish.
The math on hole size is Circumference = 3.14 X Diameter; so a hole with a 6-inch diameter has a circumference of about 19 inches, while a 10-inch hole expands to 31.4 inches. We can get just about anything most of us might normally catch through an 8-inch hole. We can get just about anything most of us might abnormally catch through a 10-inch hole. Still, when I fish for lake sturgeon I cut three 10-inch holes side by side and trim them with a spud bar.
Fish like bluegills and crappies are taller than they are wide. The diameter of the hole is more important than circumference. A 15-inch black crappie is about 8 inches tall. I've not measured the height of any huge bluegills, but like a 15-inch crappie they're a handful, so probably about 8 inches tall.
There's an ice fishing hole size for every situation and it's best to error slightly on the large size — just in case. May the fish you catch this season be too large for a 10-inch hole.