How much space does Apple music offline songs take up?
Apple Music offline songs are stored in this directory:

/Users/[your-user]/Music/iTunes/iTunes Media/Apple Music


A normal Song with a duration between 3:45 min and 5:02 min is about 7MB to 11MB large. rule of thumb is to calculate 10MB per Song and you are on the safe side. 

For Example a Maroon 5 song:

12 This Summer.m4p
(8,5 MB on disk)
/Users/bMalum/Music/iTunes/iTunes Media/Apple Music/Maroon 5/V (Deluxe)
Duration: 03:44