I suspect there's no easy answer to this question. Remember that the PS3 has no idea that you're using an HMZ T2 and is asking for a screen size, which you don't really have anyway. It's true that reducing the PS3's screen size setting increases the 3D effect, but you can definitely go overboard given your actual screen size. With my 90" screen for example, setting my PS3 to 10" gives far too much separation in a game like Uncharted 3 unless I significantly turn down the in game 3D slider. Fortunately with an actual screen I can physically measure the separation (the ideal is a maximum of 6.0-6.5 cm for objects which should appear to be off in the distance).
I suppose with the HMZ T2, since both "screens" are already separated approximately the distance between your eyes, the test should be for far off objects to appear to be in roughly the same place on each screen? That is, if the peak of a mountain in the left eye appears more rightward than it does in the right eye (when viewed with one eye), then there's not enough separation. If it appears more to the left in the left eye and more to the right in the right eye, there's probably too much. I don't have the device myself though (but curious), so that's just a guess.