This is my previous build, the Tiny Traveling Tea House. It is based on Tiny House Japan by Tagami Haruhiko. It was built into a 5x10 trailer, but I would suggest trying to build one on a 4x8 or 5x8 trailer. I would also suggest building it to only a height of 4 feet, rather than the over 5 foot tall structure I built- this may sounds small, but it's built for you to sit in/lie in, not to walk around in! All that space above my head went to waste. A shorter/smaller profile would make for easier traveling too! Of course, if you just want to build a backyard retreat, then build to whatever height you like.
I am going to make new plans for this build, with the idea of building the doors on the side to make for easier access. In addition, I will design it in mind for making towing easier.