Also, once the lines are full with no air, the master cylinder size should be plenty to operate the brakes. I've never heard of anyone saying the master cylinder was to small. What you will get with different cylinders is more or less pressure at each wheel due to the piston sizes. You have a leak or air in the rears, fronts sounds like an adjustment issue.
Till you get the air out or leaks fixed, tighten each adjustment as tight as possible so you are not chasing two problems at once. If you can get the pedal tight with very little movement, then you can back off each wheel adjustment and see how the pedal reacts.
