Dog Trailer Wheel size - for commuting/touring I'm upgrading my dog's ride from our old beat-up kid Burley, to a trailer made specifically for pets. I've found one with aluminum frame that looks affordable &amp; comfy for my pooch.... Question: would 16" or 20" wheels be better for a variety of road conditions, including off-road?  Don't want it to be so bouncy for him that he gets sea-sick. Thanks in advance!
I don't have a dog, but that means my trailer passenger can speak. 

Specifically she can complain about the rough ride compared to the child seat on the back of my bike.  That's with 16x1.75s pumped up to a pressure intended more for her comfort than my ease of riding (compared to a seat with minimal padding over the top of a 700x28 at 80psi).  Trailers are much rougher than bikes, so you want to minimise that.  Go for the bigger wheels.  The higher centre of gravity isn't nearly as much of an issue as it would be on a bike, because trailers  are so much more stable.

In addition to the choice of wheels, remember that it's much harder to avoid bumps when you've got 3 tracks to keep off them, two of which are behind you and not in line with your body.  With cars around you often can't keep the nearside trailer wheel out of all the potholes. Your passenger, whether human or canine, is likely to get bumped around more than you.