Many oil painters work at a larger size but still alla prima. Time spent will depend on the painting and how you apply it. Most often, in fact, those who work larger think of it as having more room to throw paint around. If you want to have relatively the same amount of detail as what you've been doing, except larger, then it will likely take you longer - but not always. If you more or less just scale up a small painting to a larger size, then you don't necessarily need to add more detail. On the other hand, if the objects are approximately the same size but there's more stuff (like pulling back a camera) then there'll be more work involved. You could also do a combination of large brushstrokes (say in background) and have other areas with finer details.
I tend to work slightly larger, but still relatively smaller than many other artists I know. Typically around 18-24" or so. To do the scenes I prefer smaller than @ 9x12" is too confining to me, unless it's like a larger painting with a small section cropped out of it in close-up, if you will.
As far as working in stages, my system these days is to rough in the undertone for the whole painting at the start, and then bring each area up to near completion as I work around the painting. It requires me having a fairly clear idea of what the whole painting will look like beforehand, but that's what I prefer. Others tend to want to allow the work to evolve more as they go.