Select the cell or cells
Go to the Home tab
In the Cells group, click Format
Choose Row Height to make the cell taller
Choose Column Width to make the cell wider
Drag the row boundary down to increase height
Drag the column boundary to the right to increase width
Double-click the row or column boundary to auto-fit the content
Use Wrap Text to make text display on multiple lines
Merge adjacent cells if you want one larger cell area