First off, what platform are you streaming to?
As far as 48fps being in the picture... don't. If your stream is going to be viewed on a computer monitor, 48fps is a jittery mess when played back on 60hz, and there's no good conversion (4:5 is the ratio, and that has a very jerky feel to it). Stick to either 30fps or 60fps.
For the resolution, that depends on what platform you're streaming to, and what's supported. If you're streaming to twitch, there's absolutely no limitation -- you can do 1600x900, 1536x864, 1280x720... all of those would work just fine. The difference you'll run into is in the encoder's ability to compress the frame, and that's where you just need to experiment. You might find that native 1600x900 looks best. Or you may find that the encoder can do a better job with 1280x720. Do test recordings and use your own judgment.
There's other questions to be answered when it comes to this though as well... such as what bitrate is available to you, what encoder you're able to use, what quality preset you're able to use, and whether the platform you're streaming to does transcoding.