I’ll try and work in metric here. I’m about 167cm or so, maybe 50-55 kg. 120lbs, whatever that works out to in kg. My small wave board is a 5’4"x20.5. Looks like this (during the hot coat, at least)
It carries thickness all the way to the rails, which are sort of oddly shaped but it works nicely for me. Light nose flip to avoid pearling and an otherwise flatish rocker. I could defintiely go shorter. I totally agree with fiberglasshi that you want to go short. You don’t need length in small waves. When I ride this board, people ask me how big it is then compliment me on my paddle speed. Since you’re light like me a small, wide and sort of high volume board will probably make things really easy for you when it’s small.
I’ve never measured the rocker on mine, but it’s defintiely not 4" in the nose, it’s less. Tail has 2" at most. Of course, it depends on all the other measures. Like fiberglasshi said, make it flatter rather than super curvy potato chip style.
My personal opinion would be to go wider than 18.25". Mine is very wide, 20.5". It could probably be a bit skinnier. But I was going for something very fast and slidey and I like wide boards in general. It’s 17" at the tail, 15.5" at the nose, one foot from each end. I don’t know what you like, but 18.25 is a width I’d use on a board for better waves, unless you have an wide tail like a tomo board or something. And even then, 18.25 is skinny.
If you want projection, maybe keep the wide point forward, or use an outline that will help with that. I want to say straight rather than curvy but I’m not a hundred percent sure that’s correct. Or play with fins. Mine has the wide point back since I’m generally not a big drawn out turn person. I wanted it to be easy to throw around in tiny surf and pivoty. Maybe somebody who knows more than myself will chime in. I’m sure there’s plenty of tricks to give little boards projection you’d lose from making them short, aside from fins.
Basically, my opinion is get something short, wide and with some volume. You’ll gain a huge advantage, especially since you’re light. 5’6"x18.25" sounds like my performance shortboard for good waves. You also really won’t lose much by going wider. It’s not that much harder to go rail to rail. And if you’re just hopping around and going slow on a skinny shortboard, you’re not going to be doing rail turns anyway. With the wider, thicker board, you get to use your speed instead of spending all your time making it. I’ve had that board for like three years now, and of the four I’ve made for myself it’s my favorite.