Just to clarify:
720 (as in 720p) is a resolution, not a frame rate-- but a frame size.
Stream key is the string of digits you use to tell OBS how to broadcast to your channel if you aren't using one of the new service integration features where OBS sets this up for you. It doesn't have anything to do with resolution, frame rate, bitrate, or keyframes.
There's no such thing as a "stream key rate" so it may be difficult for people to respond using this term.
Usually you would set the resolution (or frame size) of your stream to be the same as your Output resolution in OBS.
YouTube does seem to have a feature where you don't have to tell them in advance what resolution you will be streaming at, so you could theoretically change resolution in OBS from 720 to 1080 and back again between sessions without informing YouTube and presumably they would adjust on the fly to the appropriate resolution. That description is in the YouTube help page that Bayarola linked above:
If you’re using Events, you can choose a variable resolution stream key to gain the benefits of Stream now. You can also specify your desired resolution and framerate manually.