I don't know that it's the only way to do it but what I have done in the past is edit the information produced after you select the 'Insert Image' icon enter your URL for the image. If you look in the information produced within the [IMG] tags you will see a width and a height near the end. The width is a number with a 'w' preceding it. The height is a number with a 'h' preceding it. If you change those numbers your image size will change. I inserted the same card image in three times and changed the size so you can see it below.
jeff