Aaron,
The girl referenced in that vignette you cited above was a 20-year old virgin from the countryside who had been deliberately (and maliciously) lied to by a classmate. The best way to avoid this situation is simply by being entirely honest.
The subject of casual dating is discussed at great length in the thread
Casual Dating and Sex in China.
The point you are raising, however, is actually different from the one regarding casual sex: namely, how do you know whether you are sexually compatible with the person you are about to marry if you have never had sex before marriage? Of course, it's a question that doesn't necessarily apply to Chinese girls only.
It seems to me that "sexual compatibility" comprises a combination of several factors in which physical "goodness of fit" only plays one small part. Sexual compatibility involves many other things such as spiritual compatibility, physical chemistry (for example, whether the girl's natural scent attracts you), and mental compatibility (meaning whether you two are on the same "wavelength").
With the exception of "mechanical compatibility" (how well the "parts" fit together, which really shouldn't be a problem barring a major difference in height), all other aspects of sexual compatibility, as I have defined it, would be very clearly known to you in the context of an intimate relationship that had never been sexually consummated.
As for the mechanics of sex itself, there are a lot of couples out there in the world who live happily together with a significant discrepancy in height and weight. I imagine they have found a least one position that works for them.