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Note that on AmigaOS and compatibles this can only be set at initialization time. You cannot change fonts for existing objects. Also, on AmigaOS you must also set Area.FontName if you set this attribute to a numeric size. If you pass one of the predefined font sizes (see above) on AmigaOS, you must not set Area.FontName, though.
Also note that on AmigaOS Area.FontSize can't be used with widgets of type Label class. Use Text class class.