How can I find out?
Maybe I should upload some screenshots on online storage to make space on my rMBP.
Are you asking how much space you have left on your Mac or how much a particular folder is taking up?
If its the first click on the
logo in the top left of your screen.
Select about this Mac, then more info.
Then select the storage tab and you'll get a breakdown.
If its the second, just right click on the folder and select get info. A tab will open up with the folder size (among other options).