Very useful app!! It would be nice if you could add a box dimensions feature when you can see the number of pixels on each axis (the one that is saved in the xml file). Also it would be very nice if the user ccan modify the box acording to these dimensions. A dimenetion checker would also be very useful (eg. if one axis has less than N number of pixels it will pop a notification).

Machine learning with Tensorflow requires both x and y to be more than 32-33px. So this tool would save a lot of traing on crap data time!