(1) Google drive supplies the size of the folders in their sync preferences. Apparently Dropbox hasn't figured out how to do that yet, which is unfortunate. For those of us who have a smallish SSD drive, we want to be selective as to which folders we sync. Lack of the folder size is an enormous hindrance. This is not hard to do. You can just run a background server process that continuously calculates the sizes of all folders. It could be a feature given out first to paid subscribers. Needs to be on the main folders and sub-folders.
(2) The Dropbox web site has this ridiculous "calculate folder size" feature. One literally has to select the folder and then click Calculate Folder size. I was calculating folder sizes in beginning programming class in the 1990's.
A general feature that would be very helpful for managing folders and space would be a histogram tool that show space usage and activity. How does one decide to sync a folder? Well, it's based on folder size, space availability on my drive and the amount of activity on that folder (frequency of updates) and folder/file age. For example, pictures from 2011 that I rarely look at don't need to take up space on my device. A more sophisticated heuristic-based UI would help enormously in making decisions. Easy to do, and a huge improvement.