How to size my image so its a set size when someone with the link hits print on an A4 printer.

Iv made an image and iv got a link sonic can share it with my friends.  But...I want it to be a certain size, so that when my friends open the link..they can just press print on their A4 printer, and the image is automatically printed at around ¼ of A4 page size... im using my android phone and the app. Im not very technically minded at all..so step by step would be greatly appreciated. 

Someone said that when my friends get the image via my link..they just set to print at 25%... but I don't know the size of my image to begin with. Please please help me out guys. Thank in advance

 Angel-Rose