class MyModelAdmin(admin.ModelAdmin):

    def formfield_for_dbfield(self, db_field, **kwargs):
        field = super(ContentAdmin, self).formfield_for_dbfield(db_field,
            **kwargs)
if db_field.name == 'somefield':
    field.widget.attrs['class'] = 'someclass ' + field.widget.attrs.get('class'
        , '')
return field
Resize fields in Django Admin