Hope the below suggestions worked for you.

Once the image is in the post you can click on it and edit it. In there you can reduce the size of the image. I also like to just use the Paint Program if you are on a PC and I have a training here about that: https://my.wealthyaffiliate.com/training/simple-tips-on-using-paint-to-crop-resize-edit-images

If you can't edit the image by clicking on the edit image icon as you've said, there are a couple of other things you could do. One would involve writing the size you want into the image code in the text editor, but perhaps an easier way would be to delete the image and re-insert it.

If you delete the image from the page it will still be in your media folder in WP. You can then re-insert it. After placing the cursor where you want the image, press the add media button. You will be taken to your media files. Click on the image. On the lower left you have your insert image into page button. Above that there are some options including sizing. You can select thumbnail, small medium or full size (medium may not be there if it's a small image.) Pick small or medium depending on what you want, then add it to the page.

If it's still the wrong size let me know & I'll explain how to do it manually.

Here try this video
https://my.wealthyaffiliate.com/training/how-to-insert-an-image-into-website/a_aid/4d126388\

You're not dumb at building websites. How many of your friends have got as far as you have right now? Do what Kerri (Digger) suggests below - that's how I would do it too.

All the best, Mark

I am a newbie my self, but what I do is go to the page or post and click on the left hand corner of the image ( a box will appear that you can make the image small, move it to the right, left etc) ,,, reduce the size and click save/ or update I forget what it say exactly :-) Hope this helps.