Expect to spend about $200 for a good one.
Or buy a kit and build one yourself.

Wike Shopper
Wike Kit
BikeRev
Bykaboose

There's also the Burley Nomad.

Two things:
- a bigger wheel size is not that important: in fact, the bigger the wheel, the higher the center of gravity you will have, making your trailer tipsy.
- pay attention to the hitch mount: this, for me, is what separates good trailers from bad. I have the BikeRev Shopper and the Wike Premium Double child trailer. Both have unique hitch mounts, but neither of them will fit on my Brompton (the Bykaboose looks like it will, but I'm loathe to buy another trailer). If you ride a regular mountain bike with quick release wheels, then you should be OK.