Jim, I only have experience with the alcohol-filled holding plate of our Isotherm system...and it isn''t their large plate, either. So I can''t offer a first-hand comparison with e.g. Technautics plates which, as I understand it, use a glycol solution. Having said that, a friend rebuilt his Westsail icebox (perhaps that will give you a feel for dimensions and shape) with >4" closed cell foam on all sides and then installed a Technautics system. The system worked well but he was consuming 65-70 amp/hrs/day, sometimes more, so this was certainly no silver bullet re: efficiency. I would gauge his dimensions to be 50% larger than our approx. 5 cu ft box (which was the one drawing the 30-35 amp/hrs/day). These measurements were all taken with a Link 2000R (both boats) and in Bahamas/Florida spring/summer weather with water temps around 80F.
Re: what you can expect from a freezer, my impression is that it depends a lot on how clever you are, as well as how big the box and the freezer section, of course. When I installed the Isotherm, I riveted up an aluminum box not quite as deep (top to bottom) as the holding plate, and a bit narrower. I riveted on a bracket which hung over the top of the plate and slipped down its back side. This left the metal freezer box fitting next to (and touching) the plate, so lots of transfer. That''s worked great and no one has bigger/clearer icecubes...but in reality, I now realize I''m not using the coldest part of the box and would, at the least, build the freezer box all the way down the plate to the bottom of the larger box - that''s where the cold air hides!<g>
Our little freezer box has two vertical aluminum ice cube trays and some minor food items in it; it''s capacity is probably 5 or 6 of those ice cube trays. Not very big...