To reduce the actual size of the worksheet to 10 columns and 100 rows, you can follow these steps:

1. Select the row number below the 100th row that you want to delete.
2. Hold down the Shift key and select the last row number in your worksheet.
3. Right-click on any of the selected row numbers and choose "Delete" from the context menu.
4. A dialog box will appear asking you to confirm the deletion of the selected rows. Choose "Delete entire row" and click "OK".
5. Similarly, select the column letter to the right of the 10th column that you want to delete.
6. Hold down the Shift key and select the last column letter in your worksheet.
7. Right-click on any of the selected column letters and choose "Delete" from the context menu.
8. A dialog box will appear asking you to confirm the deletion of the selected columns. Choose "Entire column" and click "OK".

This will delete all the rows and columns beyond the 100th row and 10th column, respectively. This will reduce the size of your worksheet and make it easier for the conversion program to read every active cell.