When you insert an image in an original post, clicking on the "Visual" tab gives you a full preview, including the image. You can click the upper left corner of the image and get to a control panel that lets you adjust the image size at your leisure until you're ready to hit "Publish."

In comments, it's trickier. Few of these tools are available. I have trouble inserting images in the original comment, and usually have to post the comment and then work on it in the (slightly) more robust comment editor.

I post the image using the "img" button, and then the resulting image command will look something like like this:

(img src="http://blogspot.com/fish_2.jpg"; alt="" /) (I replaced the arrow things with parentheses)

If the picture's too damn big add this text right after the "alt" statement and before the final backslash: width="300" The number can be anything you want, and I usually take more than one try to nail the size I want.

It's a pain, but it works.