Image dimension might differ based upon the size defined by your theme. However, the default thumbnail size of wordpress are.
Thumbnail (default 150px x 150px max)
Medium resolution (default 300px x 300px max)
Large resolution (default 640px x 640px max)
Full resolution (original size uploaded)
You need to set the image size first, you can do that using Simple Image Size plugin. After you have set the new dimension and named it. You can select that thumbnail size from our plugin.