Never scratch built a cat but have built a number of scale hydros, I mostly use 1/8 and 1/16" light ply for the structure, 1/4" for transoms on the bigger boats.
I'd look at something about 36", good size for a pair of 6s 4 or 5000mah batteries run in parallel and maybe a leopard 4092 somewhere around 1300kv or so, a T-180 esc would work but you would be at the voltage limits, a high voltage esc may be a better option but expensive.
If you go around 42" you are looking at an 8s setup to keep it reliable, then it costs a lot more for everything from chargers to servos.
My personal preference is for boats around 30-32" running 4s setups, I just find it to be a very cost effective size to run. The hydros are a little bigger but they have lower drag so draw less amperage, they are 1/10 scale.
My dream is a 1/8 scale boat, going to have to budget over 1000 for that build.
Hope this helps a bit.