Actually it depends on where you are and whose question you're answering.

In this part of the world (where Ponters and I live) a quart is indeed 40oz ie 1/4 gallon or 2 x 20oz pints. But I take your point that Ponters may have wanted to know what the reference to a quart actually was.
The question really is whether a 15oz bottle is sufficient and I'd say it is given the suggestion that it needs to be 1/3 - 1/2 of a quart. That's 11-16 oz in the US or 13-20oz here