The issue of sizing bicycles is so confusing that a lot of makers have abandoned using inches or centimeters and now just size them from small to extra large; you'd clearly fall into the latter category. In general, frame sizing is based on the height of the frame, minus the wheels. Hence, a 21-inch road bike fits my 30-inch inseam. Mountain bikes have a "shorter" frame so you can dismount more easily, making up for that with a taller seatpost and handlebars. They're usually about four inches shorter than an equivalent road bike; sure enough, my Marin mountain bike is a 17-incher. But still, every maker has its own little sizing tricks, so it's difficult to say "buy an X" and solve your problems. You live in Seattle, though, so I should think you could find a bike shop where you can try several bikes at once and find one that works.
That aside, I'm inclined to say what you want is a true mountain bike, which doesn't necessarily mean spending a fortune. I don't want to suggest that hybrids are delicate, because they're not. But your 270 pounds will put plenty strain on the wheels, suspension, and so on. Most good hybrids, such as the Cannondale Adventure 500, sell in the $500 range. A decent hardtail mountain bike will run a little higher, but not by a mile. Take a look at something like a Giant Iguana ($500; www.giant-bicycles.com) or Raleigh's M600 ($700; www.raleighusa.com). Both are nice bikes with aluminum frames, good front shocks, and decent components.
For smoother riding on pavement and good trails, have the dealer swap out the stock knobby tires for something like an Avocet Cross, which has a recessed tread that runs smoothly on hardpack, but also grips pretty well in soft dirt or sand.
Support Outside Online
Our mission to inspire readers to get outside has never been more critical. In recent years, Outside Online has reported on groundbreaking research linking time in nature to improved mental and physical health, and we’ve kept you informed about the unprecedented threats to America’s public lands. Our rigorous coverage helps spark important debates about wellness and travel and adventure, and it provides readers an accessible gateway to new outdoor passions. Time outside is essential—and we can help you make the most of it. Making a financial contribution to Outside Online only takes a few minutes and will ensure we can continue supplying the trailblazing, informative journalism that readers like you depend on. We hope you’ll support us. Thank you.