adjustable persistence size

Originally created by @numbat on #17394 (Redmine)

There could be at least 2 reasons to want to allow the user to chose the size:

USB disks are getting huge, some people may want a 3rd (or more) partitions for other things
A user reported having an issue with a 1TB. They received errors so they had to reduce the size of the remaining free space (#17393).

Edited May 15, 2020 by numbat

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