Ah, found out myself. For anybody who is interested:
Open up flickr.php, search for this line:
's' => __( 'Small square, 75x75' ), // s small square 75x75
You will find it two times, add the following line below both of them:
'q' => __( 'Large square, 150x150' ), // q large square 150x150
and you are done. At least if you only use the widget. ??
If this gets added to the next release it would be even cooler so I won’t mark this topic as resolved yet.