Enabled by rP23022/D2340, the GUIbase.h file can be renamed accordingly with GUIbase.cpp to CGUISize.cpp.
See summary at D2340 for the reasoning and the reference to the few words on IRC with Vladislav.
The reason to rename the class CGUISize is because it does contain a size specified in GUI syntax, because it is the type of the "size" properties and some size members.

While at it, we can avoid the duplicate 0 0 100 100 constructor with a static getter,
and avoid temporary instances that are copied on success.

Notice that the FromString function already has its own temporaries that are reported on and discarded upon failure and copied upon success, and states that in the header comment.
So the copies in CGUI.cpp are unnecessary.
Notice that SGUIIcon is only constructed in four places (all four with new), and those are always assigning the 0 0 100 100 default value.
Hence we can construct them already with that value and avoid code duplication and copy assignments.