If you are simply inserting an AutoShape across the cells and adding text to it - Right Click on the "Text Box" and Select Format Auto Shape. (Be careful to select the outline of the shape and not the text within the shape or you will not have all the options).
Select the Properties Tab and Choose the "Move and Size with Cells" option.
The box should now stay above the selected cells no matter how they are sized. The box must completely cover a the appropriate cells for this to work. If there is a smidgin of the cell showing underneath, those columns or rows showing can still be changed in size without the text box changing in size.