What size Generator and what voltage and amps to power a large 3 phase well pump?
250 ft head for the well = about 125 psi.  You typically run the local system at 60 psi, so 180-200 psi.  

http://www.engineeringtoolbox.com/pumps-power-d_505.html

says 15 hp.  If we use the usual 1000W power to produce 1 hp, that's 15 kW.  Your Granddad's 25 kW generator wasn't overkill. (It's actually 745 w/hp, but you've got a bunch of inefficiencies nibbling at you too.)

Alternatives:  

Newer pumps are considerably more efficient.  Gould has a line that has a built in VFD (variable frequency drive) You feed it single phase, it converts it to 3 phase, and also adusts the speed of the pump so that it doesn't cycle nearly as often.  Do they have 3 phase VFD?  Never looked.

If the place was set up and working with 3phase, then keep it.  If you are going to do a lot of repairing/converting to solar to reduce the generator run time, etc, then getting a largish single phase generator, and converting where needed to 3 ph using VFDs may save you a lot of aggravation.

With an 8 inch casing, you may be able to run a windmill pump as well as your electric one at the same time.  If the windmill fills a lined dugout, your power bill is cut in half.

Generators don't loaf well.  You want to avoid having a generator under less than half load -- the engine runs too cool, and you are using too much fuel chasing bits of metal around in cicles.

Having a generator house with temperature controlled louvers, or exhaust fan will help keep the engine at it's best operating temperature. 

If you are really concerned with efficiency, then have several units of various sizes, that cut in and out to match the demand.  This is a difficult control set up, well beyond my expertise. 

I would consider consulting a power engineer for this setup.  If you have to replace the generator, and are thinking about the pump, then a few hundred bucks for a professional opinion may save you a lot of time and money.

Meanwhile, you can get a feel for how things are really operating by renting a 25 kW 3ph generator, and put a recorder to figure out how much of the time it's running, and how much power it's using.  An engineer will want this information anyway.

Or Buy the generator at an equipment auction, if you need to resize later, sell it -- probably get what you paid for it.