MJC,

As you have also mentioned, many other things affect the "optimum" size of a tank.  Customer locations, siesmic and wind zones, foundation, ringwalls, etc...

Several codes/specifications have their own spin on tank sizing.  API-12F sizes them for you, and to be able to put the API nameplate on, the tank diameter and height must be that size.

UL-142 also has some "guidelines" for tank sizing with respect to gallonage.

As for API-620 and API-650, as well as most other codes, they are more concerned with the pressures the tank walls see during operation.  However, Appendix A, Tables A-1 through A-4, of API-650 give some sizes of tanks with respect to shell course height.

When a customer asked us to "size" the tank for them, we would first look at the following:  internal pressure, vacuum, wind and seismic zones, and field erection or shop built.

If the internal pressure and vacuum were 0 psi and in a high wind or seismic zone, we would size the tank with a large diameter diameter and low sidewall.  When internal pressure or vacuum was present, along with no, or low, wind or seismic zone, we would tend to make the tank taller.  There was a middle of the road mentality when we had all conditions or a mixture of internal, vacuum, wind, or seismic.  Hopefully some of the reasons we did the above are evident, if not will elaborate more later.

For shop built tanks, diameter was governed by permitting and road access.  I have designed shop fabricacted tanks with odd D/H ratios of .104 to .600, as well as several in between. For field erection, it generally went from .600 on up.

Other things that I always considered, regardless of how the pressures were finally defined or the seismic/wind zones were determined was the labor factor.  Welding on anchors, sub-arcing seams, joint preperation, erecting, and crane operating, to name a few played a very large role in costing.

So I would put all of the above factors together, along with customer feedback, etc... and come up with what I thought was the optimum size.

I hope in some sort of wway this has helped.

Scott

Here is another possible consideration to throw into the mix.  If you are going to agitate the tank with a propeller type mixer, the height to diameter ratio can impact the mixer selection.  Generally, for a top-entering agitator with a vertical shaft, a height to diameter ratio of 0.80 to 1.2 is optimum for mixing. Mixers can be successfully applied in other tanks, it just starts to affect the selection and increase the cost.  
For side entry mixers with horizontal shafts, large diameter tanks with relatively short sides are preferable.
There are special cases and many other factors that can affect mixing, but it is something to think about in design, along with the many other factors.