What size frame would I need for a 27.5 or 29er mountain bike if I am 5' 9"?

I write the same thing on this website day after day after day... Frame sizing from one company to the next is NOT universal. Sad but true. How did you come about to purchase your current Trek? Did you TEST RIDE it first before the sale? I hope so. And would the new bike be a hardtail or full suspension?

The best (and only way IMHO) to purchase a new bike is by going to an authorized dealer, consulting with a salesperson & TEST RIDING a few. If you go to the Trek website & look around, they have SIZING & FIT charts for just about every bike they make. This will give you a rough estimate. Here's a Marlin 5.

http://www.trekbikes.com/us/en_US/bikes/mountain-b...

The 13.5" and 15.5" frames come with 27.5" wheels. All other have 29" wheels. I would purely speculate you'd need the 15.5" frame. Again...don't buy a thing unless you can test ride it first.

The frame size you need doesn't depend on wheel size. If a 16" 26" bike fits you, the same size frame will be needed for a bike with bigger wheels