Question: Can't set the user-defined paper size.

Answer:

(The dialog "Page options" in the designer, the paper width and height edit boxes always disabled). 
FR uses only papers supported by the printer driver. If the driver reports that it can support the user paper (paper number 0x100h in the WinAPI), FR allows these edit boxes when user selects the "user-defined paper". Some drivers use another paper number for the user-defined paper - FR can't works in such cases.