Apple - How can I find the size of multiple folders?

cmd+alt/option+i will give you info on multiple highlighed items.

There are 3 forms of "Get Info":

1) Standard "Get Info" achieved through Command+i which can potentially create multiple windows for each item selected.

2) "Get Summary Info" achieved through Command+Control+i which uses a single window to show combined info about the selection:

3) "Inspector", like @iskra mentioned, achieved through Command+Option+i, which is similar to 2) except that it updates the info dynamically based on your selection.

Open a finder window and set the view to list view. Press command-J and select "calculate all sizes" then click "use as standard".

Now folder sizes will show in your finder. I don't think it is possible to show sizes for multiple folders, but this might be a good workaround.