Excel cells will hold your text as you enter them, however, if the text is long, it can run into other cells. You will need to make the Excel cells fit the text.

To make all cells the same size in Excel and Google Sheets, select all the columns/row and then click on the Format tool to change the Column width or Row height.

Merge cells and adjust row height or column width to make cells bigger in Excel.

Select as many adjacent cells in a single row as you want to have columns under the header, starting with the cell you want to use as the header itself and moving to the right.