Select the entire worksheet by clicking the triangle at the top-left corner between the row numbers and column letters, or press Ctrl+A
Go to the Home tab
In the Cells group, click Format
Under Cell Size, click Row Height
Enter the same row height value and click OK
Under Cell Size, click Column Width
Enter the same column width value and click OK
To make all cells square, set the row height and column width to matching proportions
To apply the same size to specific cells, select the range first before changing row height and column width
To make all cells fit their contents evenly, select the range and use AutoFit Row Height and AutoFit Column Width
To copy the same size to other sheets, repeat the same row height and column width settings on each sheet