This is definitely an interesting topic and realistically the thinner the better too get that surf style feeling.
The problem is it comes down to ability and everyone is different due to fitness and age.
The pros surf some serious short narrow and thin SUP's but are pretty much underwater when paddling.
The average punter needs something about 10' x 30" 150-160 lt to start and then progress from there as they get supitis.
Most short boards are less than 20" wide and Mals are generally about 22-24" wide.
It is almost impossible to get a board that is 30" wide or longer than 9" to go rail to rail and even if you watch traditional long boarders they pivot off the tail and engage the the last third/half of the rail.
The beauty of SUP's they have glide until you go short and fat. The long thin SUP's have glide but generally cant stay close to the pocket.
One thing I have found through trial and error, demoing and burning a hole in my wallet is, the tail shape and thickness combined with fins make a huge difference to the performance of the boards even if they are 30" wide. No it wont go rail to rail like a short board but it will engage the back third/ half of the board and get that part of the board on a rail and turning well.
At the end of the day its good to be challenged and to be able to go shorter and thinner but its no fun swimming.
Its a balance dictated by ability and bank balance