You have two options mate.

1 = Assumption
2 = Experience



Assumption works like this:

Your engine is making 200bhp, and is doing so from 13psi absolute pressure

Therefore its making 15bhp per psi you put into it.

So at a bar of boost, it will have 29*15 = 435bhp


Then you have to take into account the restriction the turbo offers, the extra heat in the engine etc.

So you knock a bit off, say 10-15% or so.

So you estimate that at a bar of boost you are going to get 380bhp or so.



That sounds very high for a 2 litre engine running only a bar of boost, but obvioulsy the thing is you told me it was 200bhp from 2 litres in the first place, which is also very high, a cossie motor N/A on a typical restrictive inlet (and hence 13psi absolute versus potentially 14+ for TBs) and on the cams it runs etc even with the compression higher would be lucky to see 150



Now you know you are going to need therefore just over a bar to see your required 400bhp.

So you find a map with 2 bar absolute on the left, and 40lbs/min (its roughly 10lbs per BHP) along the bottom landing you on a decent effeciency island and hey presto you have found a turbo in the right ballpark on the compressor side.
Then pick a fairly standard sort of turbine housing, like a .63 as its not a massive power output per litre so surge isnt likely to be an issue.