While the link you gave gives you correct width, your theme may be responsive and may have different width on different screen sizes.
The best way in this case is to find the maximum width your theme support and then make images 2x that.
To find the maximum width:
- open your website in Chrome. Right click the blog column → inspect.
- Now, toggle device mode on. See this link: https://developers.google.com/web/tools/chrome-devtools/device-mode/
- Once done, select ‘Responsive’ from drop-down at top. Enter a big width, like 1900.
- Now follow the steps in the tutorial you mentioned.
This should help you with images on all screen sizes.