Setting a custom design size for a UITabViewController (in Storyboard)

I want to modify an app that currently has a UITabBarController as its initial view controller.

The goal is to have a custom status bar in the top area of the screen that will always be shown no matter which tab is selected. The current UITabBarController may not use the full height of the screen:

/----------------------------\
 |Custom Status bar (50 px)   |
 |                            |
 |----------------------------|
 |                            |
 |----------------------------| ---
 |                            |  |
 |                            |  |
 |View of the selected tab    |  |
 |                            |  |
 |                            |  |
 |                            |  |
 |                            |  | 
 |                            |  smaller height of the UITabBarController
 |                            |  |
 |----------------------------|  |
 |Tab bar                     |  |
 |                            |  |
 \----------------------------/ ---
 

I use storyboards. I cannot set a (design) size in the Size Inspector window even with simulated metrics size set to 'freeform'.

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