Standard Yukon front and door speakers are 5-1/4". Most aftermarket speakers will require an adapter bracket.
Also, there are custom made kick panel enclosures for the Yukon.
Installing new speakers in the doors is a rather easy process. You just need to remove the door handles, door lock and window switches, and the door panel, remove the speaker, disconnect it, plug in the new speaker, and then reinstall everything you took off in reverse order.
One online aftermarket retailer,
www.crutchfield.com will send you an "install guide" tailored to your vehicle when you purchase from them. They will also include any brackets, adapters, or wiring harnesses needed, usually at no additional cost.
In order to be more specific, additional information on your vehicle would be required. Is it the Yukon, Yukon Denali, Yukon XL, or Yukon XL Denali and does it have the Bose system with or without OnStar, and does it have the OEM subwoofer?
With this information, Crutchfield or many other online retailers can help you select exactly what you want.