What board dimensions should i be looking for?

Hey guys. Was wondering, what +- dimensions should I be looking at board wise for a surfer like me (+-78kg, 6 foot)? I'm a very average surfer (you could still say beginner). I can ride down the line, move up and down a little bit and thats about it
I currently own a 7'2" x 20 x 2 3/8 Lost (what i surf mostly) and a 6'.4" x 13 3/8 D.Girven Rusty (I was given this board so I'm just hanging onto it for the future). I am happy with the Lost buoyancy wise and ease-of-catching-wave'ness but did i maybe go a bit unnecessarily big? My biggest issue with the board is duck-diving. I can duck diving it pretty well (considering it's size) but if I mistime once, it drags me soo much and on very duck-dive busy sessions, by the time i get to the backline i feel like my arms are noodles. Duck-diving it is basically like benching at the gym :\
Like just in general, what dimensions are generally suggested for a surfer with my weight and height. Just want to make sure I'm not creating extra stepping stones by using the "wrong" board, surfing is hard enough as is

Like just in general, what dimensions are generally suggested for a surfer with my weight and height. Just want to make sure I'm not creating extra stepping stones by using the "wrong" board, surfing is hard enough as is
