automatic saving map areas for use off line

Question on how the app saves areas for off line use (and the space this takes). When I zoom in on different areas and turn on airplane mode it has saved the map details of these areas. I'm very surprised it seems to work this way, but I'm wondering the following: what is the size of the area it saves when you zoom in (is this a certain quare km?) and what does it mean for storage on your smartphone, or does it saves a max amount of mB for this? 

PS: Just started to use the android app. Love the speed and simplicity of this app (currently mainly using Osmand).