Steps to reproduce:
1.Open a page with VE
2.Add an image which has an full size less than the default size for example: http://en.wikipedia.beta.wmflabs.org/wiki/File:FS_barnstar.png
It has a full size of 111x107 px which is less than the default size 220x212 px
3.Now if you switch to set to custom size , it will show Size values are invalid as it is larger than the full size.