Font Sizes, how do they work?!

[OniLinkPlus]

I know that font size works by setting the height of a specific character to the chosen height in pixels, and scales other pixels accordingly, but I forget which character this is.
Also, if I set the font size to 12, what's the height of the tallest character in an average font, say Arial?

[Laurent]

The character size is the maximum size that a character can have. Which one it is depends on the font. But why do you want to know that?

[OniLinkPlus]

The character size is the maximum size that a character can have. Which one it is depends on the font. But why do you want to know that?

I'm dealing with finicky sub-window titles, and I need the height of the top bar to be just right for the title text. I'm working on a custom GUI system with sub-windows, although right now I'm tempted to just ditch the custom sub-windows and let users of the library just use sf::RenderWindow.