I would estimate the power transformer requirement as;
a) the total heater power, plus
b) worst case HT/B+ power required by the output stage. This will be the rated output power x2, plus a bit for snake bite, say 20-50% to taste. Transformers are generally available in stack sizes such as 20, 30, 50, 100 VA (volt-amps, read "watts"), so whatever figure you come up with, say 37.6VA, you will go to the next highest available stack, say 50VA.
Using a transformer with a higher rated HT/B+ winding will produce a stiffer supply with less sag, and which will generally run a bit cooler.
As a designer normally all I'm really interested in is using a tranny that has ratings in excess of my expected worst case conditions.
That's the quantitative, but I'd take notice of what Phabb says about the qualitative - no point in having it a bit louder if the cost is having it sound like a chain saw.
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