swrossSWRoss, Go with a size large based on your size.
As for a "slouchy" fit we designed the beanies to have a nice deep fit and a good size range that works even if you step up a size. The design is such that extra fabric will bunch or slouch in the very back of the hat. Then you can still have that smooth look and fit at your forehead while slouching any excess material to the back.
If you really look at it our heads are NOT a perfectly round domes. We've got mostly flat foreheads with a curve into our hairline, followed by a lightly rounded crown and more of a large bulge making up the back of our head.
So instead of making a simple symmetrical design like you find with most beanies we designed a hat that was able to fill the curves of your head based on the pattern shapes of the hat. The front/top pieces are split down the middle, while the back has an extra piece of horizontal material that either fills out or bunches depending on how it fits your head. The headband is doubled up and snugs nicely to your brow while the top panels smooth along the curves at your forehead.
In the end I think we came up with a beautifully ergonomic beanie designed to fit the actual shape of your head.
Evan