James-
In general all boats have a maximum motor size/weight rating, as for normal powerboats, larger outboard motors cost and weigh more than smaller ones and use more fuel, so it is very common to not use the maximum size the boat is rated for.
Batteries are a complex subject, I'm going to skip over standard lead-acid deep cycle types, they just are not a good option for deep cycle trolling motor applications, except perhaps for a temporary, test things out situation. they are heavy and do not tolerate deep cycle discharging well, even if they are supposedly designed for that purpose.
Li-Ion and AGM (absorbed glass mat) are the best options. Li-ion has the highest power density, highest cost, and lowest weight plus longest lasting: 5000 cycles (think phone/laptop/tablet batteries):
60amp-hr 20# $480 3amp-hr/# $8/amp-hr
80amp-hr 25# $680 3.2amp-hr/# $8.5/amp-hr
AGM is what most people use, heavier but much lower cost. these are sealed so they can't leak but are easy to damage if you don't charge them properly, always use a high quality charger specifically designed for AGM and recharge them ASAP. Most are expected to provide at least 350 deep cycles if you care for them. (I used the sale prices for these which made the cost per amp-hr jump around)
35amp-hr 26# $90 1.3amp-hr/# $2.6/amp-hr
40amp-hr 31# $130 1.3amp-hr/# $3.3/amp-hr
75amp-hr 57# $180 1.3amp-hr/# $2.4/amp-hr
I have used these Power-Sonic AGM for my kayak sonar batteries for 7 years now (5amp-hr and 7amp-hr sizes) and they have performed really well.
As for one large or multiple smaller batteries, the AGM are roughly the same amp-hr/#, so it really doesn't matter. The Li-ion gives you slightly higher amp-hr/# if you go larger but at an even higher cost per amp-hr. In this case, if I was looking for about 70amp-hr, a pair of those 35amp-hr AGMs is how I would go. I run a Minn-Kota 55# Riptide/CoPilot motor OFF the bow of our 16' powerboat and power it with a pair of 40amp-hr AGMs under the front bench seat plus a 30amp-hr in the stern that also starts the outboard.
cheers, roger