We should probably have several errors:

• "Size can be up to <width>x<height> pixels." (if too big for media)
• "Size must be positive whole numbers." (if not valid, e.g. negative or "hello")
• "Size is limited to <maxsize> pixels." (if SVGMaxSize reached)

Should those block the 'apply' button, or just give an indication that the size itself won't be applied, giving the user the option to apply the rest of the properties?

(In reply to comment #2)

Should those block the 'apply' button, or just give an indication that the
size itself won't be applied, giving the user the option to apply the rest
of the properties?

The second – see bug 60602.