firebox basket help please...

If you go to big it will stop up the air flow CG's Fire boxes are two small as it is I know I have one been cooking on it for 6 yrs. you can go 12.5 X 16 X 4 burn time is dependent on cook temp. If you want long burn times Build a UDS That CG will cook real good on Splits that is why it is Called STICK BURNER They aint meant to run on charcoal.

Don't get discouraged I Start out with about 2-3 lbs of lump & two splits then 1 split every 45 minor so to maintain temp and I cook at 300 95 % of the time. You can use charcoal but Splits are easier and more efficient and make better tasting BBQ. Most folks who complain about a CharGriller being a leaky fuel hungry beast of a cooker that won't maintain temp without constantly having to fiddle with it. I have $30.00 in materials to mod mine it runs even temps across the entire surface and eats 1 split every 45 min.
Offsets are all about a good flow from intake to exhaust, having the right sized fire box to support the cooker is critical. The one on a CG is 85 % the size it needs to be so it suffers if you stuff to much stuff into it. It also does not have enough draft under the fire to get a good burn in it's original configuration. The base of the fire needs to sit in the center of the intake so there is a place for the ash to go without smothering out the fire. This cooker requires a Small Clean & Hot fire.
If you go putting a Big Charcoal basket in it you block up the air flow around the fire reducing the draft. That leads to stale smoke and a smoldering fire and creosote deposits on your Brisket, Ribs Shoulders or whatever your cooking.
This is mine in action

Puffin Sweet Blue Baptizing the Brisket

+1^^^
I also do just splits. I sometimes have a fire going in a fire pit next to the smoker to have some hot coals to put in the smoker - depending on the length of the cook.

I put my problems with the char-griller to inexperience with smoking. I've turned out pulled pork that people nearly fought over, and the brisket never made it out of the house. Still have to battle the temps a bit, but I'm getting there.