When you need to select a whole column, click on the column letter, and it will highlight the column. You can then manipulate the content or the formatting of that cell.
To select a whole row, click on the row number and it will highlight the row so you can make adjustments to fonts, etc.
Selecting Rows and Columns using Spacebar Shortcut
I also recently learned about a keyboard shortcut involving the Spacebar.
Ctrl + Spacebar selects the entire column
Shift + Spacebar selects the entire row
Using these shortcuts will save us from having to highlight and drag our way through a long worksheet.