Electronic – Standard enclosure sizes

I am working on a project and have decided that I will design the PCB so that it fits a standard enclosure size, as was suggested here. All electronics websites sell some variety of enclosures, and then there are sites like OKW (and others) which have every variety imaginable.

I don't want to get locked into a specific company or catalog for an enclosure. Just as LEDs come in 5mm and 3mm sizes, what are common standard enclosure sizes?

For example, radio shack has some excellent inexpensive boxes, like a 5"x2.5" for $4. However, Mouser doesn't have any 5×2.5 boxes. The closest box digikey has is 5×2.25. Well I don't want to depend on radio shack exclusively for these things. So what would be a "very common" dimension for a box? For "big boxes" versus "small boxes"?