I was wondering what size roll pin holders and roll pin punches I would need, for building an AR-15.

1/8" roll pin punch and holder for the rear trigger guard pin.

3/32 roll pin punch and holder for bolt catch. You can buy specialized punches for this from Brownells that are profiled to help avoid scratching the receiver, though these are not needed.

5/32 pin punch for trigger pins.

This is what you need for a lower.

If you need to remove a gas tube, the proper size for that is a 5/64 roll pin punch.

Read the sticky in the build it yourself section...they have pictures and everything.  Punches are needed more for taking things apart.  You will need them for gas tube and FSB removal.  But to put a lower together they are not needed.

You can get a nice set from SAW if you want a top shelf armorers tool kit.

M16/M4 Armorer's Tool Kit

Roll pin holders are for sissies. Or guys that do roll pins for a living. A cheap Harbor Freight set of RP punches has lasted me for years. Not just for AR builds either.