Arghk. Did you ever know you had read something and just can't remember where? I recall this measuring was something I had looked up and used in an old article on Ex Votos. What I actually wrote was:

The candles lighted by the faithful and burning before the Blessed Sacrament, relics, or images of the saints in any church are ex votos. The origin of the use of votive candles is obscure, but from earliest times there was a symbolism attached to their use. As the incense sending up its cloud of fragrance was a symbol of prayer, so the candle consuming itself was a type of sacrifice. Grateful clients in the Middle Ages "measured themselves" to a particular saint by setting up a candle the same height or weight as the person who had received or desired some favor. Although ex voto candles are burned in almost all Catholic churches and shrines, some are more noted for this than others. At the shrine of the Virgin of San Juan in San Juan, Texas, so many votive candles are lit by the pilgrims that a shrine employee is kept constantly busy removing these from in front of the altar and taking them on a little cart to a large room set aside for them where they can finish burning.

Unfortunately I didn't put a citation i that article. I seem to recall I used that information in my first book on sacramentals and got it from something about candles. You might try the old Catholic Encyclopedia. I also seem to recall it started in the Carolingen area. If you need me to look further, email me off list and I'll check for sources.