I am going to build or get built a new track pursuit frame. It won't be a pure pursuit frame as I want to do some omnium events as well, but it will definetaly be more like a pursuit bike. How does one go about sizing a bike like this. Aerobar lengths is probably preference, but when measuring reach, should that be about your mid-forearm? I don't really have a clue about the sizing or geometry of a track/pursuit bike, so everything is appreciated.
Ask your frame builder or an experienced bike fitter. Pursuit positioning is about finding a medium between maximum power output and aerodynamics and finding a position you can actually ride in without too much discomfort.
Not something that can be easily done over a forum.