What is the best way to avaluate sustain? I am particularly concerned about evaluating the the sustain in the octave starting at the C above middle C. I've tried playing individual notes(without pedal)and timing how many seconds until the sound is inaudible, but have thought that this may not be such a good way because the length of the "useful" part of the tone(which would seem to be very subjective) may a be more valid criteria to measure. Are there any standards for "good" sustain for different size pianos ? Should I expect larger pianos to have better sustain than smaller ones in the octave starting at C above middle C, or will greater sustain only occur in the bass of these bigger pianos(with their longer strings)?

a larger piano will definately have a longer sustain..both due to the increased string length and the larger size of the soundboard... as well i find that older pianos...pre ww2 often have a very long sustain...most likely due to the greater amount of wood used in their construction...vs. some pianos today which use more metal, and fibre board and stuff like that...i play a 1928 heintzman 7' which sustains like crazy...
seems to me your testing for it the right play...sustain is the length of time it takes for the note to decay to inaudibility..i cant think of another way to test for this.