Why go short, if you are comfortable on a lb then if it's bigger just ride what your more comfortable on.. Going from a log to a SB is tough, especially if it's larger. The likelihood is you won't paddle hard enough, end up in the lip and get flogged.
As a rough guide...
9x16ishx22.5x13.5x 2&3/4 as rough dims.
Flat nose. Hard edged rails. Bit of tail kick.
Put your head down, paddle hard, keep your back foot over your fins, stay low and grab your rail if it's backhand.
And grow a set

Failing that a 7x21.5 egg type is a pretty good compromise, you still get the paddle power. Can duck dive it.