But let's say for some reason, they decide they can co-exist with the SU(which is impossible but for the sake of argument).
Would they be able to repel an identical normandy invasion with all of their available troops?
The Germans get anally raped by the Red Army in 1942 or 43, which will be equivalent to the later war Red Army but with enormous amounts of industrial and agricultural land that IOTL was captured by Germany and millions more soldiers and possible soldiers, along with workers and farmers to support the home front. The Red Army will have superior equipment, and unlike in 1941-43 they'll have the total strategic initiative along the entire front. It also has brilliant commanders like Vlasov who IOTL were killed or captured.
Okay, lets say that some ASB makes it so that there's an immutable barrier between German and the USSR, or Stalin is otherwise unable to hurt Hitler (the total lack of support infrastructure would slow things up, since without Barbarossa the USSR won't be getting the millions of trucks it got at a knockdown price OTL), Can NAZI Germany put off a 1944 Operation Overlord?
It's possible... but I don't think the Allies would even try Overlord in 1944 if the Soviet Union wasn't involved.
How much bigger do you think it would have to be?
It wouldn't be the Overlord of OTL, certainly, which was predicated on the German distribution of forces. If some or even most of what was in the East in OTL is in France, then the invasion has to be wildly different to work. And by different, I mean way, way bigger.
But it wouldn't be on that scale because they wouldn't have Russia, just France, Poland and the Balkans.
Or we could go with the slightly less ASB method, where Hitler realises he might well lose to Stalin, and so doesn't really try, instead building a massive Maginot type line along the whole German-Soviet border, and meanwhile Stalin knows the Germans will put up a fight, and thus waits it out for Germans and British+Americans to exhaust themselves before stepping in.If that's the case than this should be moved to ASB.
And then they land the troops the day after they nuke the beaches. As the invasion force collapses due radiation poisoning, American scientists begin to understand the effects of radiation in human beings.Or a nuke dropped on enemy forces near the beaches before an invasion. And, another on them as they are trying to reform and get their act together.
The huge forward leap the U.S. had in the nuclear arms race meant if they were willing to use the bombs on military or civilian targets they win.