I found it tempting to search just because I surprised why I didn't ran into the samples with the sidebar navigation mostly on iOS. After I reviewed a bit, I assume that is probably (the idea behind it described below besides this) because Apple even makes the explanation of Navigation Bars covering landscape only.
That made me think of the possible known proportions of their own manifactured product is relatively more appropriate for the landscape navigation instead of portrait design naturally.
What Might Be The Purpose Of This?
As you might notice from the design guidelines like iOS 10 Design Guidelines for iPhone and iPad, since the screen width is smaller than the vertical space, it's more likely that one can ran into more samples for the portrait navigation I suppose. Or simply navigation sidebars with the feature
slide-in / slide-out.
A Bit Research on The Issue
I wanted to share beneficial extra links which I ran into when looking for further explanations and hope one can take advantage of,
Are navigations fading into sidebars?
Adapting UI to iOS 7: The Side Menu
Creating a Sidebar Menu Using SWRevealViewController in Swift
How To Add a Slide-out Sidebar Menu in Your Apps
As a result I found out that sidebar navigation libraries for iOS not exist but mostly deprecated or simply not many source or components described on the iOS's resources itself. Hence there are components for macOS.
And the answer is?
I found the answer after a short review on a Stackoverflow question and here another covering tutorial not with the same topic and it might be a bit harder to find if you're not a coder. Even the UIViewController's subclass SWRevealViewController reveals it, it might be changed as you wish as mobile development turn into platform independent structure with a lot of different involvements.
P.S.: I ran into more examples slide-in/slide-out sidebar navigations that mostly covers the %90 of the screen (just letting a bit of the ex-action screen reenable by the user) so that might be a spare idea to further evaluate. And there is no guideline that I found saying the navigation sidebar's exact size.
TLDR; 260.0f or 220 (not fixed)