Hi @HollyBerndt ...
Although it can be done, I hardly ever recommend resizing images directly in the RCE (Rich Content Editor) of Canvas. The reason for this is that if you had uploaded a large image (let's say it's several MB in file size), if you then load that image onto a page in your course and then resize it (scale it down to something smaller), the file size never changes. The image size may change, but the file size does not. And, if you were to enlarge the image, you may start to see the image become pixelated and distorted. So, what I always recommend is to do any image editing (including scaling the image up/down) in an offline setting using image editing software on your computer. For example, you could use the Windows Paint program (or the Mac equivalent). Then, re-upload that file to your course, insert it into your content page, and you're done. No need to re-size because you've done it offline.
I hope this will help a bit. Sing out if you have any other questions...thanks!