The original McKenzie River boats were all made from plywood with 16' foot side panels and 48" inch bottoms.
In architecture or in computer graphics, if you want to scale something up so it is proportionately the same but bigger (or smaller) you use a constant multiplier for all dimensions. That is how Autocad and SketchUp work.
For instance, if you want to scale a boat made with a 16' foot side panel up to a larger boat using a 18' foot side panel, that's an increase characterized by a 1.125 multiplier. If you started with blueprint dimensions for the smaller boat and multiplied each and every number by 1.125 you'd get the same boat, scaled up to a little bigger. The original Mckenzie blown up to use 18' side panels would have a 54" inch bottom.
16' is to 48"
17' is to 51"
18' is to 54"
19' is to 57"
20' is to 60"
The above dimensions would all make the same boat, albeit at different lengths. Proportionately they are the same.
By contrast any boat made with a 19' foot side panel and a 48" inch bottom is, by comparison to the original McKenzies, a substantially different boat. The Briggs with its 19' foot side panels and 48" inch bottom is the longest narrowest river dory ever made. That isn't necessarily bad. It is what it is.
The Montana Riverboats Honky Dory started off as a boat made with 16' foot long side panels and a 56" inch bottom. That is a wide boat. There are dimensions for an HD made with 18' foot side panels now too. That's the same boat scaled up to be bigger. That one has a 63" inch bottom.
People like the HD. The closest thing I've heard to a complaint was Larry Hedrick saying it was so easy to turn you had to pay attention and keep your hands on the oars at all times. It ferries well too, which makes it easier than other boats to sneak out of a big wave train. With rafts you can't even think about ferrying out of a wave train. They turn so slowly you end up sideways to the big waves at just the wrong time and then over you go. With a raft you pick a line and point it straight ahead, and maybe twist a little left and right to hit any diagonal waves head on. But forget bout leaving. Once a raft is there in the wave train you have to ride it out.
Hard-chined dories are more nimble than rafts. That's why people like them so much. Some dories are more nimble than others. Some are side-to-side tippy and slow to turn. Others are more like sports cars by comparison.