Sometimes I find that when I paste an image, Discourse chooses weird dimensions. I find that big images may appear way too big, and small images may be scaled up and turn fuzzy.
I stumbled on a way to manually change image sizes while I was looking in another Discourse-powered forum: you can add a percentage to the inline image code. Drop in your image, then follow the image dimensions with a comma and a percentage, like this:
(You can adjust the pixel dimensions too, but I find it easier to use a percentage and have the computer do the math for me.)
If there is a way to adjust image size via the GUI, I could not find it.