Hi RogerS,
be careful if you are considering getting a trailer made up at home because I think there are new regulations (or soon will be) regarding these. I don't think the law stretches to existing trailers, and I'm not sure how anyone could prove your trailer is new or existing anyway. I was offered a caravan to convert a few months ago and researched the topic on the web, but decided to decline the offer. Many diy enthusiasts said the end results were rarely worth the effort and cost because caravans are not designed for weight carrying..
Regarding size, I made one 6ft 6ins long by 4ft wide with a ladder rack at the front, and it has been so incredibly useful that I would not want to be without it (the trailer, not the ladder rack, although that's well worth having too). The only thing about the ladder rack is that I wish I had made it removable, in fact I'm going to make that modification. When carrying stuff in wet weather it would be handy to be able to throw a tarpaulin over the entire trailer and lash it down all round. As it is, the tarpaulin sticks up around the rack and some wet always gets in when travelling. Also make sure that the internal width is 4ft or 1200mm, so that man made boards will fit in widthways, even if they do have to stick out a bit. And don't forget lashing points for rope / straps (and not positioned so the rope always catches the light cluster, or obscures the number plate, as in my first attempt - sod's law.).
I used car tyres but there's a definite trend towards proper trailer tyres these days. I'm not sure if they are better, but when I made mine most people used car tyres
Finally, it's probably cheaper to buy a used one at auction, even if it needs some work, than to buy the wheels, stub axles, steel, hitch etc etc and make your own from scratch.
Hope this helps,
K