I made the recent transition from shortboard to longboard too and agree with others here.
At your weight and height a 9' would be good you could probably go 2 5/8 and 22 3/4. A bit more rocker for beachies with 2+1 set up. The narrower (22 3/4 or so maybe even 22 5/8) will be good for turning which you'll like. These dims should be big enough for you to get up on the nose too. Just run your thoughts by a good longboard shaper.
If you really get into it you can scale up length, thickness and width a bit (and address other factors like rails/rocker, fin) for a smaller wave noserider. That's my plan.
I'm 85 kg and 183 cm and went with 9.1 x 2 7/8 x 22 3/4 and am loving it