I do not have experience with motorised plane design, so note that the assumptions below may be off. Also, I rounded the calculations towards the pessimistic side.
On the plan it says that the weight of the plane should be in the range of 4.3 to 4.6Kg. Given that motors and battery packs are a little lighter today than in the time the plan was made, it seems reasonable to assume a 4.5Kg lift-off weight. Note that I would trade the reduced battery pack weight for extra flight time by using a heavier battery.
Given the huge fuselage, floats and the Clark-Y profile the sink rate in motorless flight could easily top 2m/s.
This means that in level flight the motors need to lift 4.5Kg * 2m/s ~= 90 Joule = 90 Ws. Assuming an efficiency of 50% for motor & propellor, this means a continuous input of 180W.
Given that the heuristic of 250W/Kg for sport & training models would translate into 1125W for a 4.5Kg model, the 180W seems a little ‘light’ (factor of 6!). However this is comparing level flight with peak power needed during take-off or light aerobatics, so it may still be in the ball park.
With a 4S battery pack this translates in 180/11.4 ~= about 16 amps of continuous draw. Since there are two motors, they will draw about 8A each.
The original specification calls for 3000mAh batteries (8 NiMh). I am assuming the plan was from a commercial kit, so a flight time of 10 minutes for that configuration seems to be a reasonable assumption. Which btw seems to confirm my above guesstimate of 180W for level flight. (Or as the cynic in me tells it, perhaps two errors I made cancel each other out…)
Since batteries have become lighter, it should be possible to install somewhat more capacity for about the same weight. A 3000mAh NiMh is about 60 grams/piece, for a total of 500 grams (including wires, connectors, etc). A lipo with 5000mAh also comes in at about 500 grams.
5000mAh would yield flight times of perhaps 15-20 minutes. 5Ah / 8A ~= 35 minutes. But real flight will easily half that number. And it is better to have some reserves left when landing.
What all this was really about is the size of the battery pack. A 5000mAh pack is approximately 150x55x35mm3. Though shopping around can probably change the dimensions a little, the volume will likely remain about 150x55x35mm3.
And the big question is: will it fit the battery bays that are in the design? The answer to that is no. Which is a bit of a bummer, but not too bad either. Remember that I wanted to use the battery packs to balance the COG. I can thus safely forget the battery bays as they are and create a new one, at a location better suited to my needs. But this will lengthen the leads between battery and ESC, and hence a capacitor will be needed at the ESC end to prevent induction spikes.