But there is no TSizeF type.Is it declared in some other file?
Why would you expect that? a type has a fixed size expressed in one of the integer types. types are not size 1.0000345678 size.
There will be a Types.TSizeF in the upcoming FPC 3.2.4.
I think Thaddy simply mistook "a type called TSizeF" for "a type with a floating-point size".