The container size does not matter at all, i plant about 30-50 freesias (depending on the size of the corms) every year in a container whith specs 12 inches width and height also 12 inches and all of them bloom and at the end of the season give me new corms that i store for the next season, as for ranunculus if same type of container is used maybe about 6-10 tubers i plant (depending on the size of tubers) in one container.
Plant them in a free draining soil with lots of organic matter and water only when the soil feels dry... over watering in the winter leads to rotting.
And the seed store guy was right, soak your ranunculus tubers over night and when the tubers are swollen plant them, this process hastens the growing of the plant.
split the ranunculus only when there are multiple bunches in a single bought tuber like the pic below otherwise it is not required
go to this link.
http://www.waynesthisandthat.com/ranunculuses.html