The 7 Series FPGAs Transceivers Wizard v1.5 in ISE Design Suite 13.3 shows incorrect values for internal and external data widths in the GUI when 8B10B is used.
The GUI shows the external data width selection as multiples of 16 (16, 32, or 64) and internal data widths of 16 or 32 when 8B10B is enabled. This is incorrect because the internal data width should be a multiple of 20 (20 or 40) with 8B10B when the external data width is 16, 32, or 64.
However, this is a bug with the wizard GUI only and the generated HDL code sets these correctly. This is fixed in the wizard GUI in ISE Design Suite 13.4.
For the complete data width settings, please refer to Table 3-1 of the 7 Series FPGAs Transceivers User Guide (UG476).