Since #2029, font sizes are now consistently measured in CSS points, which makes the default sizes quite similar on all backends:

• Android: 10.5pt (14dp)
• Cocoa: 9.75pt (13 "Apple points")
• GTK: 10 or 11pt
• iOS 12.75pt (17 "Apple points")
• Windows: 9pt

But we can't make them identical, because we’re following each platform’s own UI guidelines. This makes it difficult to use the font_size style, e.g. a Toga widget with font_size=11 will appear larger than the default on WinForms but smaller than the default on iOS (see examples/font_size).

I think the best solution to this is to support the CSS font size keywords small, medium, large, etc.