If they were the same size on all three, then nobody would've had to push so hard to have anyone make special Ts for our rigs. We'd all just go to Home Depot for brass 5/8" Ts or order up some aluminum or stainless ones off Amazon or somewhere.
I'm not 100% about NBS, but mine was 3/4" off the firewall, 5/8" to the water pump and 1/2" to the rear heater. So, when ordering from JTR/Stealth, "Size A" would be 3/4", "Size B" would be 1/2" and "Size C" would be 5/8".
As the others have said, the OEM plastic ones are fine. But, if you do a lot of work in the area and are often at risk of cracking them, metal ones with hoses are much more forgiving. You'll just have to accept the aesthetics of multiple clamps in close proximity. I used constant tension clamps from GM, Honda and Ford (salvage yard digs) and positioned them all in line to look as clean as possible as well as easily accessible with long needle-nosed pliers.