Folding Bike Wheel Sizes

Looking at Folding Bikes for inner city commuting, and there are a wide
choice of wheel sizes - from 12" to 14" to 18" to 20". Given the
parameters for the "foldability" and understanding the smaller wheels
obviously will provide the most compact package - what are the riders'
preferences here ? If you want a good Folder but you want a good ride,
what is the optimal wheel size ?