Geissele is 1.41" for co-witness and 1.57" for lower 1/3 (based on an email response I received from them). ADM is 1.52" for co-witness and 1.69" for lower 1/3.
I don't remember the exact height of my riser but I prefer to have it co-witness dead center.
Most are offered as a "lower 1/3 co-witness". The lower 1/3 co-witness riser that came with my red dot, the front post was barely in view. Looking at other offerings, it looks like it depends on what sight you use. This is from a quick search.
Looking at the sig red dot
if its the romeo 5, it will be absolute co witness.
I simply cut off the front sight and went with 45 degree offset backup sights since I like my red dot to sit as low as possible for a consistent cheek weld!