I know this is possible in Xcode (click + button next to Font property)
But in VS for Mac, there is no + button next to the Font property.
Is it possible to even do in VS, if so, how do you do it in the Storyboard Designer?
It seems this is a known issue: https://bugzilla.xamarin.com/show_bug.cgi?id=56977
It seems this is not supported well, if at all, in Xamarin iOS Designer at this time. I think using XCode IB is your workaround for this at this time.
Once I added different font sizes for different size classes in XCode, I opened the storyboard in Xam iOS Designer and the correct font sizes were shown based on the device view selected, and then there is a little "..." button next to the Font property, but clicking on it shows the different size classes, but the wrong numbers for the font size settings (all show the AnyxAny font size). So do NOT edit the font size in Xam iOS Designer after setting it up in XCode as Xam iOS Designer will then apply the edit to ALL size classes.
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