Front master cylinder replacement

I have a 98' Dyna Wide Glide that I have purchased a 4 piston front brake caliper to upgrade the old 2 piston caliper. My question is...what size bore should the new master cylinder have to get the full potential from the 4 piston caliper....and is it possible to get a master cylinder with a bore that is Too big.? I've called the local harley dealers and they dont have any thing in stock and I dont want to order it until I'm sure of what I need. Any advice here would be greatly appreciated. Thanks in advance.

The same as what's on the bike now. single disc 9/16 bore and dual front brakes I think is 11/16 bore. The bore size of your M/C will be stamped on it, you'll see it when you take it off the bars.

so.....you're saying I can go from a 2 piston caliper to a 4 piston and I don't need to change the master cylinder.? awesome....thanks.....going fast is alot more fun when you can stop....thanks again

When I changed my single puck Evo front brake to the 4 puck TC style I used the stock 9/16 M/C, then I upgraded the switches to 96-up style which would be the same as on your 98, I used a M/C from a 2005 single disc TC which was the same bore,9/16...The TC's have a 4 piston set up.
There is more volume of fluid in the dual disc because of the extra brake line and the second caliper,thats why 11/16 bore M/C is used.
The 9/16 will provide enuf pressure to operate a single disc set up provided you got all the air out of the system.
A differential bore 4 piston caliper will work better than the 4 piston TC calipers because the piston gets smaller the farther it is away from the brake line and the smaller piston needs less pressure to squeeze it than the larger one close to the line and have a more even squeeze on the pads, TC's pistons are the same size.
The TC calipers are a major improvement over the Evo calipers but they're not the best, a differential bore multi piston caliper will provide more even pressure to the pads and better braking.
You don't need a larger M/C until you add more calipers.
When you hop up your engine , it's a wise move to beef up the brakes.