It's a topic of great debate! That's what it is! I'll share my view on it, which comes from meeting those who dub themselves nano or micro roasters, reading other people's humble (and not so humble) opinions on the internet, and finally, from my own deductions (which is why I said, "my view...").
What a nano roaster is not, is a small home-based roaster pumping beans out their basement and ventilating warm wonderful coffee exhaust out of their window well, it can be, but that's not necessarily the only way to quantify a nano roaster. The roasters I've met that call themselves nano roasters are typical very low volume, on the order of zero pounds to say ~15 pounds per week (these figures are highly debatable). They may run out of their home (like all of my businesses first started out), they may share industrial or commercial space with another company, or they might even have their own boutique coffee shop. They likely piggy-back their bean purchases with another roaster, buy from a local roaster who's larger, or even have a direct trade agreement with a small coffee plantation far-far-away. But what they're not doing is importing pallets of green beans on a regular basis. The equipment the use can vary, from a popcorn popper to a 25lbs San Franciscan Coffee Roaster. Their distribution model probably consists of friends and family, the local farmers market, maybe some web store sales, and possibly even a local coffee shop (or a couple). At the end of the day, it's almost guaranteed that the nano roaster has another full-time job and can't support his/herself on the sales they're generating... it's simply a hobby or a part-time company.
Nano Roaster Example: Agápē Roasting Project
This is where the gap widens. A micro roaster can be a small micro roaster with a handful of coffee shops they supply to tens of coffee shops in a city-wide, state-wide, or small regional area. These are the guys that run it as a full-time business. They typically order beans by the palate(s), have trade relationships with importers or direct trade relationships with a few different coffee plantations. You would be hard pressed to find a micro roaster who operates out their home (maybe a dedicated building on their home's land?). They might supply nano roasters with green beans. Often times they have their own coffee shops, or a few of them around town. I didn't mention this with the nano roasters, but both micro and nano roasters usually roast-to-order. A micro roaster may have distribution deals with supermarkets or niche food marts, but don't often have a warehouse of pre-roasted coffees.
Micro Roaster Example: Corvus Coffee Roasters
The benefit of both nano and micro roasters is that they have unparalleled quality control, since they roast in smaller quantities (than say, that ominous company which is yet to be named).
Anyway, that's just one mans opinion. Let us know what you think.