engine size shouldnt be an issue in the slightest, if you want power you can compensate for engine size with revs, just like F1 engines do.

main advantage of size in this application is spooling up turbos quickly, find yourself a big capacity high revving engine and your on a winner.

which is why i chose the M5 V10

dont see why itd need more capacity to hit 1000bhp running 2xGT40s at 1bar, infact id be suprised if it needs anything.

only thing i can see it needing is maybe compression ratio adjusted to take a solid 1bar from 2x GT40s right upto the rev limiter (on pump fuel but with a sufficiantly good intercooler or two).

a 5litre V10 like that revving to 8500 running 2x GT40s@1bar would have prob about THREE times the torque of a current F1 engine (they got just 250-300lbft ), and a bigger overall powerband too.