Searched the web all over to find the way to change the image preview box dimensions in CKeditor, with no luck.
Although https://docs.ckeditor.com/ckeditor4/docs/#!/guide/dev_howtos_dialog_windows-section-6 teaches how to change the dialog dimensions, it doesn't provide information on how to set the image preview box width and height, which remain fixed on changing the dimensions of its parent window.
Dimensions of the preview box could be changed in skin's CSS, located – in the case of moono and moono-lisa skins – in skins/<name of skin>/dialog.css
. The selector connected with it is .cke_dialog .ImagePreviewBox
.