Hi, I know this is related to this topic and this topic, however I wanted to share my personal envision for custom media sizes in order to keep this popular requested feature hot.
I 
In the picture you can see a button which allows for more screen sizes than just the ordinary. When this is clicked a small input box drops down. Here, there is a min and a max. My though is that after a new screen-size is created, a new screen-toggle-button appears on top. I think this button can be dragged and dropped between the old screen-sizes and doing so will result in min/max-values automatically updating and overwriting the user-input-value on in order to smoothly split between the different screens - likewise adjascent screen-sizes will also have their max/min updated.
Additionally to the four ordinary screen sizes, I also added another standard size, namely a screen-toggle-button which represents sized between 2k and 4k. My reasoning behind this choice is that Webflow works perfectly for all screens below HD, but its difficult to design to my large desctop screens.
I hope I added some insipiration to the webflow community. Please like and keep requesting this to the developers.
Cheers.
Magnus