Unity tends to have a harmless yet annoyingly spammed error that goes along the lines of Screen position out of view frustum (mouse pos 256.000000, 0.000000, 1000.000000) (Camera rect 0 0 256 256).

This isn't particular to Sizebox. There's no shortage of discussion on the Unity forms about the many things that can trigger this error.

Determine the causes these errors and try to mitigate around them