[_ck_]

Technically you’re not supposed to have more than one H1 on any given html page (though browsers don’t care and will render anyway). The trailing BR is a problem with bbPress’s parser.

Why don’t you use the built in ability of BBcode to do SIZE instead?

Large Text

[size=+1]text larger than default size[/size]

default size

[size=-1]text smaller than default size[/size]