Look online, most of the sites list dimensions for batteries.
Tim
Info below is from https://www.batteriesplus.com/battery/marine-and-boat/deep-cycle
Don’t know what size battery you need?
There are standardized battery group sizes for marine applications. Match up your dimensions to select the right BCI Group size.
Use this quick guide to match up your battery–approximate maximum dimensions.
Size Length Width Height
24M 10-1/4" 6-13/16" 9-3/4"
27M 12-1/2" 6-13/16" 9-3/4"
31M 13" 6-13/16" 9-7/16"
34M 10-1/4" 6-13/16" 7-7/8"
65 12-1/16" 7-9/16" 7-9/16"
Additional size may be available – Contact your local store for more information.