What are your estimates for the populations of the shadow city and Planky Town?

Not very large, according to the WOIAF:

“There are no cities in Dorne, though the so-called shadow city that clings to the walls of Sunspear is large enough to be counted as a town (a town built of mud and straw, it must be admitted). Larger and more populous, the Planky Town at the mouth of the river Greenblood is mayhaps the nearest thing the Dornish have to a true city,”

Given that towns in Westeros tend to have ~10,000 people or less, I wouldn’t be surprised if the shadow city and the Planky Town have less than 30,000 inhabitants between them.