You can try to change it in the DB:

UPDATE  `sys_transcoder_filters` SET  `filter_params` =  'a:2:{s:1:"w";s:3:"600";s:1:"h";s:3:"600";}' WHERE  `sys_transcoder_filters`.`transcoder_object` =  'bx_timeline_photos_medium' AND  `sys_transcoder_filters`.`filter` =  'Resize' AND `sys_transcoder_filters`.`filter_params` =  'a:3:{s:1:"w";s:3:"600";s:1:"h";s:3:"600";s:13:"square_resize";s:1:"1";}' LIMIT 1;

UPDATE  `sys_transcoder_filters` SET  `filter_params` =  'a:2:{s:1:"w";s:3:"100";s:1:"h";s:3:"100";}' WHERE  `sys_transcoder_filters`.`transcoder_object` =  'bx_timeline_photos_preview' AND  `sys_transcoder_filters`.`filter` =  'Resize' AND `sys_transcoder_filters`.`filter_params` =  'a:3:{s:1:"w";s:3:"100";s:1:"h";s:3:"100";s:13:"square_resize";s:1:"1";}' LIMIT 1;

UPDATE  `sys_transcoder_filters` SET  `filter_params` =  'a:2:{s:1:"w";s:3:"300";s:1:"h";s:3:"300";}' WHERE  `sys_transcoder_filters`.`transcoder_object` =  'bx_timeline_photos_view' AND  `sys_transcoder_filters`.`filter` =  'Resize' AND `sys_transcoder_filters`.`filter_params` =  'a:3:{s:1:"w";s:3:"300";s:1:"h";s:3:"300";s:13:"square_resize";s:1:"1";}' LIMIT 1;

Clear DB cache after this modification.

But it maybe some negative effect in the design, so some CSS fixes will be needed.