Well, my Paint version might be older than yours, but when I open a picture in Paint, There's a "Resize" option there (in the upper left).
Click on that and you can change the picture size by pixels or percentage. I use the pixels option. I just played around with it until I found the right number that makes the photo be smaller than 1MB and saved it.
I change the smallest pixel number (on the original picture) to be 1500 for starters. Then I "save as" using the same file name but adding an identifying extra letter or number. That way I don't lose the original. Once I've saved it, I hover over the new photo to see what the size comes out as. If it's still too big, I delete the new picture and do it again, making the pixel number smaller and saving again (with the new identifying number), check the size, etc, etc, etc.
After a while you start to know what numbers work for your photos.