I agree with the suggestions from Robert, 16-18 feet long and 22 to 23 inches wide is the sweet spot for your first kayak.

This could be a long (and interesting) conversation....
I more-or-less agree with the statement above, but...
Perhaps it's a regional thing -I'm on the Vancouver Island - but hereabouts a 23 inch wide, 18 foot long kayak is considered a 'gear carrier' for long trips ("Freightliner') - and most are equipped with rudders.
For day-paddling, shorter trips, taking classes, learning to roll, playing - the norm seems to be a boat in the 15.5' to 17' range, with a beam between 21" and 23".
I'd never recommend an 18 foot boat for a beginner or as an 'all-round' kayak.
Good 'do anything' boats around here are skeg kayaks like:
NDK Romany 16 x 21,5
Wilderness Tempest (different sizes) 17 x 22
Wilderness Zephyr 15.5 x 22.5 or 16 x 23 (depending on paddler weight/girth)

The Mariner Express is a great kayak (no skeg needed) that can carry enough food and gear for a few weeks tripping and it's 16 x 22.5
Bjorn Thomasson's Frej is an excellent boat (I love mine) - you can vary the length (mine is 16'); the beam is 21.25" and it's plenty stable.
BTW, you can't judge the stability of a boat by the beam measurement alone. However, I think most paddlers will agree that (with the same load) it's generally harder to set a wider boat on edge...and you do want to be able to edge your boat.

Fuselage boats give up some interior space with the frames, so perhaps that is affecting the beam recommendation?

And, you couldn't easily replicate some boat shapes with a fuselage boat. Brian Schultz (Cape Falcon Kayak) has used the Mariner Coaster as inspiration for his F1 (14 x 23) - something like that could be a lot of fun to paddle. (The Coaster is a legendary great boat).