Hey! First time user, have an f26 with staggered 20” setup. Where can I find appropriate sizes to switch to 20” or 21” square wheel and tire setup? Thanks in advance!

Your owners manual. There should be a chart that lists all the recommended tire sizes and PSI (tire inflation info).

You can also go to one of the online part database sites like https://www.realoem.com/bmw/enUS/partgrp?id=XW73-USA-01-2016-F26-BMW-X4_M40iX&mg=36…enter the last 7 digits of your VIN (or select correct choices)…then go to the wheel diagrams. The database will list all of the wheels offered for your vehicle as well as the tires. Simply click on the diagram to get more info.

Below is a screen shot from the f15 x5 owners manual that lists all the tire & wheel info along with the recommended PSI. A similar chart is in your owners manual that gives the correct info for your f26 X4:

^That link is pretty handy!