First of all, thanks for purchasing all our tilesets! We hope you're enjoying them and creating a lot of cool stuff. Also, sorry for the late reply :)
1) About the screen/scenes dimensions: The ideal size could vary, depending on how much information you want to include in a single screen of your game. We usually use 512x288px. That way you pixel your scenes using that dimension, but later in a gamedev engine (like Unity, as example) you fit it to a 1920x1080px screen. There is more dimensions you can use, check this link (https://pacoup.com/2011/06/12/list-of-true-169-resolutions/), there you will find all the screens sizes that correspond to a 16:9 resolution (highlighted in green).
About the characters dimensions: A normal human size would be around 18x32px. No matter the scene dimension, a human would be always like that using our tilesets. We're working hard to share a free characters pack soon, with a man and a woman totally animated, ready to be re-skinned for your needs. Stay tuned!
2) Well, outlines are a choice. We recommend it because outlines (specially clack or darker outlines) are being seen for the player as things that have more importance than other stuff that is not outlined or that have less dark outlines. That is a decision to make, but we recommend to keep outlines on characters while using our tilesets. They will fit better, and overall things are more readable using outlines. If you are interested, you can learn more about the making of pixel art assets with these online courses by Daniel Benítez (pixel artist at Mucho Pixels).
We hope our answer helps you continue with your projects. Don't hesitate to share your progress with us though here or our socials.