To select / highlight an entire range without having to drag your mouse from top left to bottom right, simply select a single cell within the range and use the keyboard shortcut ctrl + A (All).
With the ctrl key still depressed, pressing A a second time will highlight the entire sheet. Simple but very effective, especially if you have a large range to highlight.
NB! This will only work if the data that you’re selecting does not have any empty rows or columns. This will in effect split your data into separate ranges or regions - one of the many reasons why you should not have empty rows and columns in your data.