Lewis, welcome to the land of length

I'd suggest you have a read around the longboard section of the forum for ideas and see if you can try a few different longboards at your local break, i'd recommend a 2+1 fin set up with a long centre fin box so you can learn to tune your fins to what you want to do. You'll need to get out of the thruster option of 3 equal fins and look at other configurations..
You will find one interesting thing to overcome, the first few times out you will probably bury your nose to the length of your short board
Practice cross stepping on rails and board width pieces of wood and stuff so you avoid unbalancing shuffles and learn to turn the board from other positions other than the full tail.
As to size , weight and height are a consideration, the magic will happen when you understand your board

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷