Welcome aboard!
There is no particular section geared towards larger trailers. I am working on one that will use a salvaged 7' x 14' popup frame. There is a section for cargo trailer conversions you may find useful for designing your layout. Most of those are well in excess of 12 feet. I have found countless helpful tidbits of advice in various builds of all sizes. I'm sure you will as well.
Though I believe most of us here are Yankees there are many from Great Britain as well as mainland Europe, Asia, and Australia. You do have some unique hoops to jump through on your side of the pond in order to get things properly registered for road use. You'll find many useful discussions on those topics.
As you go along, be aware that we like lots of pictures!
Those of us building larger units find ourselves far more concerned with weight. Foam construction is one great solution for that problem. There is an entire section dedicated to that.
Happy building!
