I have a 1970 Dart Swinger 340 with front disc brakes and rear drum. i cannot say that the car was original with front disc. The question I have is "can someone tell me what the bore size would be on the existing master cylinder"
No, because the car is over half a century old and parts get changed (and as has already been stated, a master cylinder with equal-size reservoirs is not correct for a disc brake system—that's for a drum brake system, and using it with disc brakes can make problems).
The original master cylinder on that car had a ø1" bore. The master cylinder with the ø1-1/32" bore mentioned by mvh came in for 1971. However, the '67-'70 ø1" disc brake master cylinder has grown difficult to buy. There is no reason why you can't use the '71-up ø1-1/32" item; that's this one.
While you are paying attention to the brakes, take easy and inexpensive action to permanently fix the rear-lockup problem the disc-brake A-bodies all had from the factory; info here.