ok, so to quote Ferris Buellers day off "Anyone ... Anyone ??" ..
Not my scene, but lying in bed last night I thought about the downtube cells. if they used 18650 format you could fit 3 side by side (you might have to V them slightly, depending on the V you might just squeeze in 21700's) with two stacked over and 2 above. which would fit into a 60mm round downtube. So 7. Think Darth Vader Advanced TIE fighter shape stuffed into a tube :
View attachment 174969
It has to be limited to about 50cm length for the smallest sized frame, so with connectors and end caps you can fit 7 length ways - giving you space for 49 cells. No idea how you get around squeezing that into a 13s4p (which might not even be correct) configuration, because you can't, so we'll ignore it and use it as an approximation.
I think the existing Amflow 600wh battery has 52 cells, but rumour has it that will be increasing in capacity in the gen2 batteries.
Therefore, assuming someone can work around the problems of turning fiction into something workable, then you're looking at a 630 (wild)-700wh (limit pushing) battery at a wild guess at the top end of wild guesses.