So this is a optimised 12s7p layout drawing (found it in one of @taz 's threads) I would say the 12s7p puts you at 200 mm length and about 150 mm width.
I made a 12s6p with bulky positive and negative terminals on the side which made my pack a bit longer. About 210 mm long.
About 125 mm wide. And about 90 mm tall including bms on top
I ended up making a battery box with inner dimensions of 215 × 170 x 100 mm. This fit all the electronics nicely (battery, bms, 2 focboxes, receiver, metr pro and the wiring and loopkey and charge port.
So here you have some reference.
I found the 1200 peli is 235 x 181 x 105 mm. This is just my estimation, but I think12s7p with vescs en other electronics would be tight but I think it can be done if the pack is made nicely. 12s8p with all electronics im not so sure.