Alternator pulley size question

[86m1028]

Can anyone tell me the correct alt pulley size ?
Friend has an m998 with a 3&3/4in pulley.
I’m seeing a 5in pulley. His is damaged, any reason not to use the 5in one ?
I realize alt will spin slower but will this cause any issues ?
Thanks

 

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Bigger belt, too.

I noticed at too low RPM (below 600), the volt guage did not make it to green. After adjusting idle, it went to middle of green. I suspect if the alternator will turn slower, that the idle may need to be raised higher than normal.