We are reluctant to provide renovation advice without being able to see the situation & run some tests.

First, you need to check to see if you can reduce the heat-gain/heat-loss in the uncomfortable room.

When you do all things reasonable to educe the loads, do a manual J so you know how many cooling & heating Btu are required to properly heat & cool that room,

Quick method: The heat gain and Btu/hr of cooling is done for each room so you know what BTUH that room requires.
Use 30-BTUH for each Cubic Foot per Minute (CFM) of Airflow.
Then Select Supply duct size by cfm with 600-fpm or less velocity.
A Room requires 3000-BTUH divided / by 30 equals 100-CFM, or around a 6" dia. RD metal duct.

One 6" rd flex duct, 25' run, one 90-ell, Friction loss 0.04", 510-fpm velocity.

One 7" metal duct, velocity 562-fpm, same run length, etc. delivers 150-cfm times 30-BTUH equals 4500-BTUH to the room.

An accurate manual J tells you what BTUH that room requires. The diffuser must handle the 150-cfm airflow.

One 5" metal duct same run delivers 75-cfm at 550-fpm; therefore, two 5" ducts equal 150-cfm, close in all data respects to the 7" duct @ same length run.They're close to the same friction rate & velocities.

Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)

I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM
Times 1000-FPM = 1395-CFM. Branch ducts: 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm.

I am human, let me know if I made some major errors!
- udarrell

Here is a link that might be useful: Sizing Residential Air Conditioning Ductwork Systems

Ditto go back to the basics and figure it out the right way (manual j and manual d) or just guess

Here is a link that might be useful: Air Conditioning Repair made Easy

Thank you both for your replies.
I appreciate it.