Does an external monitor need to be a certain aspect ratio?

I’m looking into investing in a new ipad pro with an M processor mainly for it’s ability to be used on an external monitor, i currently have an older ipad so this is the only real reason i’m upgrading.

Does the aspect ratio matter on the external monitor (i plan on getting a 4k monitor with usb c port)? I obviously want something that fits the total screen, naturally. I’m not looking into buying one of Apple’s displays.