I just bought a house built in 1919 and it has central A/C installed, but I need to run new ducting as the current stuff is not insulated and is coming apart. What size, typically, should I run for the main trunk (downstairs, top-bottom feed, and upstairs) and what size should I run to the individual rooms?

Here is the basic layout of the house:
Front hall - 10x13
Kitchen - 12x13
Living room - 13x14
Dining room - 13x14

Bathroom - 5x7
Front bedroom - 13x14
Back bedroom - 13x14
Office - 8x11

It is a typical four-square lay out in the house. Total square footage is 1,351 sq. ft. The unit is barely adequate enough for the house, but it works for now. Should I run 6" flex to all of the individual rooms? All of the rooms, except for the bathroom and office, have two registers in them. The downstairs ones are 2x10's and the upstairs ones are 8"(?) circular.

Any advice would be wonderful!