Wombat " ... pros and cons of different tyre sizes to use on my yet to be built unit."
Whatever you do please use the same tyre size on your yet to be built unit.
Sorry, I'll be serious now. I had 10" tyres on my trailer. When I shredded one trailer tyre south of Blackall on the way to Cairns I could not get a replacement in Blackall or Barcaldine. I decided to head east to Rockhampton to try there rather than continue on the inland route. Fortunately, I purchased three new 10" inch tyres in Rockhampton.
On my return to Canberra, Resort Trailers swapped my 10" tyres for 13" ones to enable me to replace trailer tyres in most places. They were heavy but were good for towing the trailer behind my 4WD.
I've since put 12" tyres on the trailer.