Just got a mini 16v alternator and need to get a mandrel pully. What size should I get?

a general guide is to overdrive the alternator by around 30%

To say another way, the crankshaft pulley will be 30% larger than the alternator pulley.

Doesn't the Aerospace mandrel already have the alternator pulley? that's how mine is mounted to the balancer and then you add the rest of your accessories on after that..

The aerospace mandrel has the pully mounted flat up against the balances. My application and bracket require the alternator pully to be more forward and even with the front of the waterpump almost.

Ahh, ok. I would just try to match the size of that pulley with the one on the balancer, and then whoever makes your alternator call them up and make sure that whatever size is on the alternator itself won't over spin the alternator. When I got mine from Mark Payne (House of Payne) he told me that for the most part, these newer alternators can't be overspun so go ahead and spin it as hard as you want for the most part.

The thing with spinning it faster than required for proper voltage is the extra power it consumes.

Example one, 30% overdrive would produce 9600 alternator RPM with a 7400 RPM shift point. At a 1500 engine RPM the alternator would spin 1950. Which is at or near the alternator full output.

Example two, a 60% overdrive would produce 11,840 alternator RPM with 7400 shift point. An engine RPM of 1500 will produce an alternator RPM of 2400.