Hello,
I'm making a 2D game for mobile devices. I've my camera set as Ortographic, size 5 and resolution on Game tab as 720x1280.
When I drag a sprite of size 720x1280 and 100 pixels per unit (to be my background of my scene, it appears bigger than the camera. I saw online that a corelation with camera size, height of a block in pixels and resolution does exist, but I couldn't understand.
I just wanna fit my background without having to scale it by hand.
Thanks, @ricke44654 ,
Actually, I figured it out by other ways, but the link you shared with me helped a lot more.
Saw online a equation to calculate the size of ortographic camera and I found that, for the resolution I wanted, the pixels per unit I wanted, it has to be bigger. The equation is Ortographic Size = Desired Height / (2 * pixels per unit) . I guess everyone has checked a site that said that, but I didn’t know how to use, bechause, otherwise of putting “pixels per unit” they named it differently.
Oukei. Now I’m making it fullHD and the Ortographic Size is 9.6.
The orthographic camera size was also a bit confusing to me when I started learning Unity. After stumbling about the internets looking for answers, I happened upon this post on the Unity blog that explained it pretty well and it clicked for me. Hope it can help you as well. 
I'm making a game in 1920x1080 and the ortographic size is 5.4 and matches perfectly according to the formula. Thanks :)
– nicmarxp