What kind of tolerance are you looking for equal flows?
Effectively you will be trying to equalize the pressure in each of the branches. If you have a fixed flow you can introduce orifices to reduce flow in the higher flow branches. If it is more variable you may want to explore using a flow control valve and a flow meter on each branch.
The bigger the better. Running towers in parallel will prevent problems with shrink and swell with starting and stopping of condenser water pumps. Keep velocity below 3 fps, max conditions. It's not like piping that gets distributed throughout a building; you might be talking about a 30' run. Go for the 14" vs. the 10" (as an example) to avoid problems.