[TYPO3] Click enlarge set maxwidth

[H.E.Weenink | Aristo WebDesign]

I use an extension to show houses, etcetera. 
Visitors may click image to enlarge (please try for yourselves here:
http://www.marten-immo.de/index.php?id=1 
But somehow I cannot get it to be bigger then 400 x 400 
 
I tried global setting for image width [styles.content.imgtext.maxW] ->
Nothing
 
Where do I find this constant?? 
 
Eef