I’ve never built a tiny house from scratch but I’ve stripped and rebuilt a several travel trailers with essentially the same goal. I too started with the biggest I could pull at the time (28ft / 4 tons) but quickly realized size was not everything and downsized.

If you haven’t lived in one yet, you’ll probably find that a few inches here and there won’t matter as much as you think in the end. In my experience what really matters — including for my friends that built actual tiny homes — is how easily it can be moved. The bigger it is, the less fun it will be to move and ultimately that can be very limiting even if you already have the vehicle to do so. A 13ft tall trailer of any kind is a beast, and driving in bad weather or high winds will really suck. It’s not impossible at all, but it’s not fun either and that can slowly chip away at your will to actually take it places, especially if you plan to stop frequently. Same thing goes for really long trailers that are a pain to position in congested areas. If your goal is to be mobile, you’ll thank yourself for trying to keep it compact. I settled on a 19ft travel trailer in the end (9ft high, plus 3ft of clearance for reference) and that was ultimately more than enough room for me. It was easy to pull, didn’t wear down my truck as much, and I moved it way more often.

TLDR — If you’re planning to move around, I highly recommend trying to keep it as short and light as you can. If it’s sitting somewhere for a year at a time, well then hey go big.