Select the cells, rows, or columns you want to make the same size
To make columns the same width, select the columns, then drag the boundary of one selected column to the desired width
To make rows the same height, select the rows, then drag the boundary of one selected row to the desired height
Right-click the selected columns or rows and choose Column Width or Row Height
Enter the same width or height value for all selected columns or rows
Use Home > Format > Column Width or Row Height
To make all cells in a sheet the same size, select the entire sheet and set one column width and one row height
Use AutoFit if you want cells to adjust to the largest content instead of matching a fixed size
Copy a cell with the desired size, then use Paste Special > Column Widths for columns
Use the alignment and merge options only if you want cells to appear uniform without changing actual size