Those are just examples...
and for the record i have a turbo that fits 2.0 liter motors that makes 600 hp...
But that's beside the point. In the examples i gave i just used some fictitious numbers....
Let's look at what he has and has found.
saab 2.3L rated at 185 Hp @ 5500 rpm.
I assume (b/c OP didn't correct em) he has a 5000 rpm 305 215 hp?
donor motor rated at 185 Hp
divided by 1 turbo equals 185 Hp
times the number of turbos you will run (2)
equals 370 Hp
Donor motor one size, 2.3 liter times Redline RPM 5500 rpm = 12,650
divided by the number of turbos (1) = 12,650
Receiver motor size, 5.0 liter times redline rpm (5000 rpm) = 25,000
divided by the number of turbos (2) = 12,500
12,650/12,500 = 101.2%
a 1% chance of being incorrectly sized (too large)
going further with this:
The Saab 9-5 Sedan's 2.3L light pressure turbocharged engine uses relatively low boost pressure (maximum boost of about 8.0 psi or 0.55 bar) to deliver boost quickly,
so the turbo's are meant to last 100K miles with 8.0 psi.
so we know those turbo's (2) push 380 hp worth of air at 8 psi
8/14.7 = 0.544 + 1 = 1.544
1.544 * 215 = 332 Hp @ 8psi
332/ 370 = 89.7%
by horse power/boost an 89.7% chance of being the correct size (10% chance of being too big).
if i take 1% and 10% / 2 equals a 5% chance of being too large,
now I'm ready for you to pull out the maps and show me what they say. i'll be waiting.