Welcome Chris !
IF you have an idea of the shape of the trailer then start with that.
If you are still not shure try the Hall of Fame at the top.
The next thing to think about what all do you need in the trailer.
The next thing is make a cardboard model Full size or to some scale and see how everything you want is going to fit in the trailer.
Next is think over all those thing will not all fit in the trailer and what can you do without.
The list gets smaller.
And smaller.
Then you are ready.
Then you can deside on a size ( hint at least 60" inside wide you will not regret ).
Some can do the minimalist of 46" inside it is up to you ( make a cardboard box the inside dim. and set in it if you like it then go for it - you can do the same thing with a blanket chairs and brooms ) and put your stuff that will be in the cabin with you in the model space.
Have fun and camp like the rest of us.
Pat
Life is to short always eat dessert first.
uÊ