Here is a guillotine blade from the period, preserved in the Gadagne Museum of Lyon. It was easily dismounted ( just those three bolts) and the cutting edge is perhaps 11-13 inches: about the same as a broad axe. Whether one of the sans-culottes kept it sharp himself, or took it down to the nearest blacksmith someone else may know. But it would not be difficult to keep an edge on it.

There are numerous depictions of guillotines with much larger blades. I don't know the chronology but suspect that over the next 100 + years of using one for execution the French made changes to the machine, including making the blade bigger.