We have found a 'solution', not very pretty but it will suffice for now.
The datagrid extends from Panel, so we used the Panel properties to set custom Classes:
cls string Add a CSS class to the panel. null
headerCls string Add a CSS class to the panel header. null
bodyCls string Add a CSS class to the panel body. null
The custom class we used:
.fullWidth{width:100% !important;}
The parent divs / wrappers all use percentages to set their width and/or height.
The easyui created inline style for width is overruled by the class
fullWidth.
Setting Custom classes for the datagrid wrappers wasn't possible, it would be a welcome addition. Now we had to style these differently.