A 55-0-55 transformer will do 200~250W/8R, 320~400W/4R, and 400~500W/2R depending upon the VA ratings and filter caps. The Dynaco ST416 had this style output stage and with two 10,000µF filter caps and a 53-0-53V 1.5KVA transformer did 450W/4R. The C100 was the optional capacitance bank that brought the amp up to 30,000µF per channel, per rail, with inrush current limiting. If you read the A40 article by Nelson Pass you will see that the minimum he recommends is 3,300µF per rail, per channel, for an 8R load. This would suggest a minimum of two 27,000µF caps for a mono supply feeding a stereo amp driving 2R.
As far as VA ratings, here are my suggestions: Assume you want 200W/8R, 400W/4R, 800W/2R and are going with a mono supply for a stereo amp. Assume 60% efficency for class AB operation. 1600W out / .60 = 2,666W in. Assume a .7 power factor for a capacitor input filter. 2,6666W / .7 = 3,809W in. Assume a 50% duty cycle so 3,800W / 1.414 = 2,694W or two 1.35KVA transformers.
As a cross check we can compare numbers with the ST416. 1.5KVA transformer / 900W output = 1.667 . The factory built SuperLeach put out 600W/4R and used a 60-0-60 1KVA transformer, the same 1.667 ratio.
In reality, audio gets no worse than about 25% duty cycle. But the load can have a 45* phase angle, so the above calculations are still valid.
Can you go smaller? Sure. I have a good size pile of transformers out of Sony, Yamaha, Denon, etc. receivers that did just that.
Can you go bigger? Sure. My 120W/channel McIntosh MC2120 has a 600VA transformer with regulated supplies for the voltage gain stages. That follows my calculations too, but for 100% duty cycle. Of course it sounds like a brick house in the bass, not to mention weighing 58 lbs.