Having run UBBS, let me tell you there are plenty of reasons NOT to enable HTML. There are things that can be done (bad things) that are not possible with UBB Code.
As for editing picture size, that shouldn't be done with HTML anyway. You should resize graphics before you post them, as it speeds the load time considerably. As I understand it, the HTML size commands don't really change the size of the graphic, only the size it appears. That still equals a big download. Graphics/HTML experts correct any mistake I have made here, please.