To play, normally, fullscreen.
But by default, the first time it is launched in windowed mode.
Many games have weird resolutions or sometimes don't work right at first, if you force the first run in full screen mode and something doesn't go as it should, then it's a problem to exit the game.
A good and easy option menu. And if you can toggle it with alt-enter.
About first or third person, it depends a lot on the type of game and control.
For example, if it will be a doom type, that is, you aim and shoot, first person is better because the character takes away your vision area.
If it will be focused on melee combat or if it will have platform components, that is, it will interact a lot with the environment, better third view.
This is obviously my personal opinion, the best thing, if you can, is to implement both modes and for each player to select the one that is most comfortable for them. But the rule will be to avoid things that are uncomfortable for the majority, for example, many people complain about the platform in first person, because it is not intuitive.