I've shared this with a couple of friends, but just in case others need to know ...
here's what I discovered about how to adjust the size, and the position, of an image that has already been uploaded into a post:

1. Go to your post with the image/s, and click ‘edit’.

2. In the editor, click on the image and you will see two little icons in the upper left hand corner.

3. Click the one that looks like a little picture, and it will open up a window where you can adjust the size of your image … choose the next size up:
(my image above is set at 100%). You’ll also notice you can ‘center’ the image there, as well.
(I also found you can center the image here in the editor by clicking the image on and then going to the bar above,
where you see the right, center, and left settings ... just select the 'center' setting).

4. When finished, click ‘Update’ and that will take you back to the editor.

5. There in the editor, click ‘Preview Changes’ to check it ... if it’s not the right size for you, repeat the steps above and change the size again.
(Could be a 'bug', but I've noticed, although not every time, that I've had to do this more than once to get an image centered!)
When you do get it the way you want it, then click ‘Update’ in the editor.

Sure hope this works for you! I know how frustrating it’s been for me to figure it out!
There may be an easier way, and if anyone knows, do tell!

So far, this is what I found that’s worked for me.

Blessings,
- Deborah