Battery Recommendation

Hi,

Thought you guys might be able to answer this.... I have a 12 volt
trolling motor system and I was going to upgrade to a 24 volt within the
next couple months. My current battery is a Group 34M by Crown Battery.
It's a real brute of a battery, and it died today after 4 years of regular
service. Anyway, I was going to buy a new one, but I think the group 34 is
probably larger than what I need, and I"m worried that if I buy 2 of these
monsters, it will weight down the bow of my boat a little to much.
What size battery would be large enough to last a good day of fishing? I
was looking at the DieHard 24M's but not sure if this would be big enough.
The weight is 62 lbs opposed to the Crown's at 88 Lbs.
I don't do any heavy current river fishing, mainly inland lakes.
I wouldn't worry about the weight as much, but my boat is a 16 ft Sylvan
Super Snapper with all the decking and I'd like to keep it as trim as
possible.
My trolling motor currently is a Minn Kota 12 volt 55 Lb thrust bow mount
(digital) with auto pilot.
(hopefully) replacing with Motorguide 24 volt 101 lb thrust Rack and Pinion
(digital)

Sorry for dragging this out... In a nutshell, what's a good group size
battery to get without being to excessive?

TIA,

Doug