You probably know that you can sort Outlook items by clicking a column title. Clicking toggles between ascending and descending order.
What you might not know is that Outlook doesn’t limit you to a single-column sort. To sort by two columns, sort the primary column. Then, hold down the [Shift] key and click the column cell that represents the secondary sort group.

In the figure below, the small up arrow indicates an ascending sort on the From column.

The secondary sort, shown below, adds a descending sort by size, as indicated by the small down arrow. The difference is subtle. Only the two messages in the first group resort — sorting the 9 KB message before the 6 KB message.