I believe the 75mm was the largest. It was heavy enough that planes were known to stall firing it. It was mounted on a suspension system so that it would recoil...
The game is IL-2 Sturmovik, released a couple years back. Unfortunately, the Hs' aren't flyable It's a Russian theater themed WWII flight sim, so the predominant focus is on Russian aircraft of the vintage. It's still a great game as it includes alot of aircraft you don't see very often, including the BI-1 Rocket Interceptor, arguably the first of it's kind even predating the Komet by a year.
Vostok 7