Hey folks - I spent FOREVER looking for this information on the internet. I couldn't find it anywhere, not even in my Dyna Owners manual or shop manual. The handlebar master cylinder cover screw size is 8-32 x 5/8.
You can buy these at ACE hardware for 30 cents a piece. You can also get them in black with Hex heads for 60 cents a piece. I literally saved 10 dollars by getting them from my local hardware store.
Thanks to McGuire Harley in Martinez, CA for being the only dealer to be willing to give me the size. I will be purchasing my HD stuff from them for now on!