At the moment the image sizes can only be altered via changing the lines in question/type/ddimageortext/questiontype.php:

define('QTYPE_DDIMAGEORTEXT_BGIMAGE_MAXWIDTH', 600);
define('QTYPE_DDIMAGEORTEXT_BGIMAGE_MAXHEIGHT', 400);
define('QTYPE_DDIMAGEORTEXT_DRAGIMAGE_MAXWIDTH', 150);
define('QTYPE_DDIMAGEORTEXT_DRAGIMAGE_MAXHEIGHT', 100);

to the desired sizes.

It should be possible to alter those parameters via the moodle GUI, when creating the question.

600x400 images are not up to date and this limitation might be too small for some intended use-cases.

Also for some use-cases it should be possible to change the size of the dragimage.