Could you not control the size of <h2>, using css, and force a line break.
Depends on the exact wording of the header, and also on the exact font used. Forcing a width in pixels is out of the question
:: pause here for long digression on responsiveness ::
so you're looking at ems. The catch is that an em is a vertical measure. Unless you've set a fixed-pitch font (2 characters = 1em) you can't be 100% sure how many characters fit into, say, 12em. If your header is in a named font, check all of them-- and make sure your em measure also works for the generic fallback (usually either serif or sans-serif).