Flexbox is definitely a better structure compare to float. It’s been out for a while now, and more and more browsers are compatible with it nowadays.
The appearance of different sizes of sidebars is because you’ve set the container width of single post to 1000px, while the other pages are 1150pxand the sidebar width is set to 30% of the container width.
https://www.screencast.com/t/yGkx0tS7
I assume this 1000pxsetting is from a layout element, go to appearance > elements, check if there’s a layout element assign to posts.
Let me know 🙂