I have looked everywhere and found a few threads with no answers. What size dovetail / dovetail cutter is on the glock for rearsight? Thanks.

They are different for the different sized frames. They are also beveled with the center a smaller size than the outside. sights are not a hammer in to make fit. Most need to be filed to make a perfect fit.

I just put some Trijicons on a glock. You-tube helped a lot.

The rear sight dovetail is the same across all Glock models.  The rear sight height is some what model specific.  I have been looking for the Glock factory spec and have not found any thing.  I know one of the major manufacturers of third party Glock slides and he was purposefully unclear when I asked him rear sight dovetail cutter.  Like it's some big secret.

I believe I have figured out.  The Glock dovetail is essentially a .250" x 74° dovetail, cut ~.079" deep.  In metric that works out to 6.4mm x 74°, with a depth of 2mm.  I just cut a test dovetail in a piece of 3/4" 7075 bar stock and it is a perfect fit for a take off Warren Tactical rear sight I have.  

Tallamong is correct that Glock uses a pinched profile in the dovetails cut on their slides.  However, when cutting a dovetail to use a Glock rear sight it is not necessary to copy Glock's pinched profile.  A conventional, straight cut to fit dovetail will suffice.  And I feel is preferable.