I solved the problem by (hacking) changing NULL by the width & height I need in
components/com_k2/models/item.php in line 411 and 412:
//Plugins $item->text=''; $params->set('vfolder', NULL); $params->set('vwidth', 530); $params->set('vheight', 320); $params->set('autoplay', NULL); $params->set('galleries_rootfolder', NULL); $params->set('popup_engine', NULL); $params->set('enabledownload', NULL);