How do I see how much space uninstalling an app will free?
Short answer: The Uninstall confirmation shows each item's size and an "About … will be freed" total that updates as you tick or untick items. Empty the Trash to get it all back.
The confirmation screen shows each item's size and, at the bottom, "About … will be freed" for everything currently ticked. Untick items and the figure updates.
The space comes back fully once you empty the Trash, since uninstalled items sit there until then.