The Tracker provides real-time update information on file sizes etc. when it can do so without affecting performance. Calculating the total size of a folder’s content in real time, however, would be resource consumptive. However, there’s a nice workaround built into the Get Info panel.
Select a folder and access the Get Info panel from right-click | Add-ons or by hitting Alt+I. As you add or subtract items to/from the folder, the calculated size in Get Info won’t change automatically. But click once on the reported size and it will recalculate instantly.
This is really useful when, for example, you’re using WriteCD to burn ISO CDs and you want to keep the total size of a folder just under 640MBs.