OS X :: What Is Desktop Size On Imac

We have six separate accounts for family members on our Imac. Most work well. However, when two of our children access their accounts, the desktop background size shrinks to about 80% of the screen size. The fonts and icons also become larger. When they log off of their accounts, the appearance changes all of the other accounts until we re-start the computer. Re-start makes it go back to full screen size until on of the two affected accounts are accessed again.

I've attempted changes through the desktop appearance menus in preferences, to no avail.