From my understanding on how WordPress crops, it’s not going to stretch your image to a certain size, so if you upload an image that is 300px wide, it isn’t going to stretch the image to 400px, it will keep that width and proportionally create a height. Make sure that
Crop thumbnail to exact dimensions (normally thumbnails are
proportional)
is checked in the media section of your settings. If you’re looking for new image sizes you can use add_image_size().
If you want to size down an already created thumbnail, you can click the image to edit it (inside the tinyMCE) and size it down there. You will be given percentages and a little box on how it might look inside your content.