Rune Knight

If a rune knight is using a glaive has polearm master and great weapon master uses the enlarge ability what will ita reach be? Does the glaive add additional reach?

Glaives have the reach property, so your reach with them is 10ft, not 5ft. Don't know about scaling while a larger size

Reach. This weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for opportunity attacks with it.

So if you have a 10ft reach, with a reach weapon you have a 15ft reach.