I have a 68 chevelle malibu that i'm restoring. I was wondering if it matters what size alternator I need. I have a small block 350

it depends on what you are going to run, as in electrical equipment-(fans, stereo, water-pump, etc)
and if you intend to use the original style/type wiring harness. :)
what are your plans??

If you have no more equipment than original, a stock size unit would work, fit the existing brackets and wiring and be no hassle.

Upwards from there, if you add loads, such as ac, or electric fans, or more motor to crank, ie sb to bb, a bigger alternator is a real good upgrade.