You cannot change the number of rows and columns (well, if you save a workbook as an Excel 97-2003 workbook, extension .xls, each sheet will have only 65536 rows and 256 columns instead of 1048576 rows and 16384 columns).
But you can hide the rows and columns that you don't need.
An extreme example: in the screenshot below, I have hidden all rows from row 13 down, and all columns from column M to the right:
