So with a final compression ration of 22 : 1 and with two turbo chargers in series.

If we assume that each turbo will give us 4 to 1 and they are in series

We will an inlet manifold pressure of 8 bar

So the piston compression ratio will be 22 – 8 = 16

So we are looking for a crank stroke that will give us 22 to 1

The combustion chamber has a volume of 25ml

So with a piston of ??

But if a stroke of 95.5 gives us 19:1 (from the VW spec)