Thanks! For the pixelated look I used a custom render pipeline in Unity called HPSXRP. I also crunched down all the textures (the highest res texture is 64x64), but that was the biggest factor. HPSXRP is great for giving a PSX (and, to a lesser extent, N64)-style look, but it's also fairly restrictive in what you can do with it, as unless you want to write some custom shader code you're locked into a few different versions of the materials.
I've also had good luck with using the Retro 3D shader pack which applies the lower resolution through a post-processing layer. While I don't think the look is quite as nice as HPSXRP, it's nice if you want something that's easier to put into a project (or if you only want the effects on some scenes and not others).
Neither of those had any effect on the UI, though, so I had to do that sizing and layout by hand.