You could give this a go.
1. Make a copy of your spreadsheet (keeping the original file safe. . . in case something goes wrong.)
2. Open spreadsheet and create a new worksheet (commonly called "tab").
3. Switch to an "overgrown" worksheet that is too big because of the "last active cell)
4. Select the true/desired range of data. (ie. a1:Ak65.)
5. Cut and paste that into the same cell addresses on your new worksheet.
6. Delete the original worksheet from workbook.
7. Rename new worksheet to that of the original (now deleted) worksheet.
8. Check formulas to make sure that none were broken when moving your data. (also check/verify visual basic scripts - if applicable)
9. save file.
10. Repeat for any other worksheets "where the last active cell" has become out of sync with your true/desired data range.