Try Ctrl+End. If that puts you way past the last used row / column, you need to delete the blank rows and re-save the file.
1. Go to a blank row below your data in column A. Ctrl + Up Arrow. Come down 1 row, Ctrl + Shift + Down Arrow. Edit > Delete > Entire Row
2. Similar idea for columns.
3. Save the file
Also, if you have code, export ALL modules, delete them from the file, and save. Then re-import all modules.
Finally, if you use pivot tables and you have more than one PT using the same data source, make sure that they all use the same cache. That can save you heaps of disk space.
Denis