Hey,
1) This is not (easily) possible - I'm sorry.
2) You can adjust the preview image dimensions in functions.php - open up the file and search for:
$k_option['custom']['imgSize']['base'] = array('width'=>267, 'height'=>180); // backend preview size, if changed does not affect the frontend
$k_option['custom']['imgSize']['S'] = array('width'=>60, 'height'=>60); // small preview pics, eg for sidebar news
$k_option['custom']['imgSize']['M'] = array('width'=>180, 'height'=>135); // small preview pic for posts
$k_option['custom']['imgSize']['L'] = array('width'=>610, 'height'=>260); // big preview pic for posts
$k_option['custom']['imgSize']['XL'] = array('width'=>884, 'height'=>390); // big images for fullsize pages
After you changed the dimensions you maybe need to regenerate your thumbnails with this plugin: http://wordpress.org/extend/plugins/regenerate-thumbnails/