Fuse is determined by wire size as others have said.. I will not bother with links to charts since they have done it
Instead of upgrading grom 24's to27's.. Why not go with a Pair of GC-2 in series
Here is the math
Group 24 73 amps hours each, MARINE/deep cycle
Group 27, 95 amp hours each, MARINE DEEP CYCLE
GC-2 210-230-250 amp hours.. A bit taller.. DEEP CYCLE
(There are different sub classes. all fit the same tray but they get taller as you go up in amp hours) the amp hour numbers are for the pair, in series)
That is more amp hours than your 27's will give plus being DEEP CYCLE you can use up to HALF the battery capacity.. MARINE/deep cycle you should not use that much.