No effect on the sizing. Worth having? Depends on who you ask. I use them for 2 reasons. 1 - it moves the smoke exit down to the meat level so you get better smoke coverage. 2 - the wide opening moves the smoke/heat constriction further away from the rack, moving the hot spot away as well. Ultimately it’s arguable since there a thousands of pits out there not using them that make great product. A third reason for me is because I like how they look - but that’s just me.
It's not figured in to the CC sizing, that's why it isn't on the calculator. A lot of guys feel like it helps to even out the temps across the bottom grate so that's the reasoning behind it. Also some guys like the look of the collector on the end of the CC. You can turn the exhaust 90 degrees out the end of the CC and it will work so it's kind of up to you if you want to put in the extra work required to build a collector. I typed this to find that dave had said pretty much the same thing right before I tried to post this. It takes me a bit hen peck as my wife said so I'm posting it as well.
I just did some quick napkin math for some perspective. 24" dia x 48" cc, with a 8" tall triangle shaped collector that spans the diameter of the cc adds something like 5% volume to the cc. Not sure but that seems like a big collector too.