Can I stop offering a size in some areas, or charge more for it there?

Yes. In Settings → Service Area & Yards, open “What we offer here” on any ZIP. Untick a roll-off dumpster size, compactor, junk load or toilet unit and online booking stops offering it at addresses in that ZIP; the customer is shown what is offered there instead. Your office and drivers see a warning and can still book it, recorded with their name. “Extra here $” gives one size its own extra for that ZIP, folded into the price like the area's in-can $ (blank keeps the area's). Save to that ZIP, or to every ZIP in its city or county. Your website's city pages and ZIP check show those prices.