I have an XPS 13 which has a 3200*1800 resolution hooked up to an external monitor that only goes up to 1920*1080. How do I make it so that when I place windows on my external monitor that they are not huge and so that they are properly scaled to its resolution size? It was working on a fresh install but something happened and it doesn't fit right anymore. Thanks.
try setting your laptop display to 1600 x 900 resolution. Unfortunately, the current Linux desktop supports only one scale factor for the entire desktop, so either you scale down your HiDPI monitor, or you get double size on your external. For more info check out these blog posts:
I'm hoping the trick System76 has developed will be released and we can use it in ElementaryOS, see: https://www.reddit.com/r/elementaryos/comments/7c7x8x/pop_os_mixeddpi_multimonitor_setup_on_elementaryos/