Educate me on the Basics of Surfboard Dimensions/Shapes

I've been surfing for about 5-6 years, but only really consistently the past 2 years. I stand up on probably 9/10 waves I get and can do some simple turns and things. I've always just grabbed whatever used board I can find and never paid attention to the dimensions. Right now my main board is 6'0"x18.15"x2.15" (its the JC Hawaii SD-3). I also have a fish that I take out when its 2 foot that is 6'3"x20.5"x2.75". I'm thinking about selling the fish and buying another board. My main board feels a little narrow across the top (the 18.15). I was also thinking of getting something with a 4 fin set up, I see a lot of surfers around here with that (New Jersey). I am 5 foot 8 and my weight fluctuates between 160-170 lbs. What do you think are some dimensions I should look for?
Thanks
Thanks