What size master cylinder would be best for my setup? I have wide A calipers up front and 914/6 gt rears?

19mm

19mm I have huge calipers installed by PO and 19mm works,besides steve knows ,besides I hate working on brakes so its right first time everytime.
Done a lot of work with VW Golf brake upgrades and never run any thing much bigger than stock,might be me but hear of people going to 23mm

I have the Brembo AM calipers front and stock ones in the rear combined with the 19 mm MC. I feel the pedal throw is a little long before the brakes really kick in...maybe its something one gets used to? (my pads need bedding in too but but that car isnt road legal yet)

I cannot remember for sure what the wide A was. If it is from the Carrera 84-89 I have the identicle set up front and rear. I selected the 19mm master based on a discussion with Eric Shea. Basically the front and rear pistons diameters are the same as porsche intended so you have the same front to rear bias. The results are nothing short of amazing. Stopping power is awesome. Perfect pedal travel to modulate the braking. Excellent pedal feel.

Yes the wide A is from the 84-89 911. It sounds like you have the same setup as me. It feels like my pedal is a little spongy even though I have bled my brakes several times. I will try again.

This may sound odd however I do not know what size of master cylinder I have. I installed a new master cylinder a few years back when I had a stock setup. I do not remember what size I installed. Is there a quick and easy way to differentiate a 17mm or 19mm mc while it is installed?

You have air trapped somewhere.
Your breaks should be rock hard when properly bled.

I find that a drive on some bumpy roads after a brake bleeding seems to help get rid of those pesky air bubbles. Sure, you have to bleed the brakes twice, but the pedal does firm up nicely.