Minimum space requirement USB media

For just the PortableApps.com Platform, all you need is about 10MB.

For the apps as well, that depends on how many and which ones.

I wouldn't really say there is a minimum or typical capacity.
It all depends on your requirements and expectations.
Up to a point you gain more value with larger capacities, so I would just invest in largest you can afford if the capacity is the primary deciding factor.