There are a few apps out there to find the size of a folder but don't forget a lot of these are simply GUIs for terminal scripts. With that in mind I went looking on the best way to get the size of a folder from a script in Terminal, and it turns out DU (Disk Usage) is what you need. Below outputs the most basic information you need for a folder size:
du -h -c -s ~/Downloads/-h human readable
-c print the grand total
Of course there is a lot more to it, just type man DU at your terminal to find out more.