If you want to know which one try creating the Table of Contents described
in
http://www.mvps.org/dmcritchie/excel/buildtoc.htm
which will tell you how many cells are in the used area.
If you want to just try to fix lastcell problems then run the macro
# Why do my scrollbars go to row 500 -- my data ends in cell E50?,
contextures.com, Debra Dalgleish. The basis of deletions is all columns
without content, and all rows without content. Warning: merged cells
problems -- a merged cell has content only in the upper left cell of a
merged cell group. You could clip off unused parts of merged cells and
cells formatted as merged as in a form. It is important that you format
entire columns because formatting in partially formatted columns will be
lost from the deletion points.
http://www.contextures.com/xlfaqApp.html#Unused