Other threads have discussed the use of tu vs. você in European Portuguese and in some regions of Brazil, and my understanding is that in EP, people will often use o senhor/a senhora or drop the subject altogether, rather than use você. Following that custom, how would someone ask this question using o senhor or no subject? For example, if I were at a store trying to buy several meters of fabric or kilos of meat, how would the salesperson ask me how much I want?
In Portugal, you can put «o senhor» virtually anywhere. «Quantos metros quer o senhor?» (perhaps the most common way of saying this), «Quantos metros o senhor quer?» (a bit awkward, but still doable), «O senhor quer quantos metros?» (the implication changes with intonation with this one: if you say this with the intonation going down on «quantos» and up on «metros» it is the normal "How many do you want?"; if you say it with the intonation going up steadily through «quantos metros», it comes off as the man asked for a ridiculous amount and you are not sure you heard him correctly.
Then again, there is nothing wrong with using «você» in Portugal either, although you are right that «o senhor/a senhora» or simply «Quantos metros quer?» would probably used more often.
In Brazil, any time where "o senhor" would be used (not often at all!), it is formal enough to use the whole pronoun from what I've seen. As for storekeepers, etc, just glide through você, like "vcê". Only in really unambiguous and informal situations have i heard it dropped altogether. "Oi, amigo. Quer café?"