Hi Swarnali,
Did you checked below Importing parameters of FM GUI_DOWNLOAD.Also refer to the documentation.
WK1_N_FORMAT
WK1_N_SIZE
WK1_T_FORMAT
WK1_T_SIZE
The parameter WK1_N_SIZE description says:
"Sets the column width for value columns. If WK1_N_SIZE is not set, for every value column the output length of the field is set as width."
So it must give you proper space as per the output lenght of the field.
Regards,
Vikas