m90 blower pulley size

What you're going to make for boost pressure is all going to depend on the amount of restriction you're seeing at the intake and exhaust side. The rule of thumb is, the more you're able to reduce the restriction on the intake side, the higher your "boost pressure" is going to be. The more you're able to open things up on the exhaust side, the lower your boost pressure is going to be. Both equal an increase in power, so don't concern yourself with boost figures so much. Moving the air in and out as efficiently as possible is what you're after. I'd start with a 3" pulley and go from there. If you want to spin it a little harder and do a pulley swap, back it with a water/methanol injection set up and let her fly! Keep in mind, these blowers will pretty much hit the wall after 16,000RPM so spinning it much beyond that is fruitless. If you're having trouble finding a crank pulley, Innovators West is the place to have a custom one made.

For the record, there are one or two guys on here with custom M90 set up and they're seeing around 370-380RWHP with pulleyed M90's and your average slew of bolt ons (mild H/C/I combinations, T/B, Exuaust, etc).

The blower kit on my 4.6L Mercury Cougar utilizes a 3rd Generation M90S (basically an S-ported version of what came on the '94-'95 Thunderbird SC) and it makes enough power for me to see 12's in my 4,000lb, automatic, IRS equipped MN12 Mercury Cougar. Pretty good considering the car in stock trim ran high-15's.