Small turbines aren’t efficient anyway - too much surface area per volume fluid flow, correct?
I tend to think of it in terms of Specific Speed (Ns) and the specific diameter (Ds) which is what you're referring to.
And for most companies the big one is Ns because it tells you what machine you want. The GE guys did the right thing because it's a radial machine, the specific speed will push you there for a small gas turbine; unfortunately it will also push you to very high physical speeds of the rotors. For a centrifugal compressor that means a heavy mass spinning fast = high stress.
But all that's been overcome in the last 40 years or so, and low thrust jet engines that are extremely efficient are now common place....mostly in cruise missiles!
Good ol' Barber-Nichols has a good paper on this:How to Select Turbomachinery For Your Application
And of course GE / Pratt / Honeywell / Williams all have performance codes that just spit it out on demand...