... what size font would be good for each (streaming and big screen)?

I don't beiieve that there is a rule or standard for font size for opening or closing credits; certainly, I've never seen such a rule/standard. Indeed, a font size for one typeface will result in a substantially different size on the screen than the same font size selected for a different typeface.

In the end, it's your creative decision. Have a look at TV/cinema/streaming programs/movies (DVDs/BDs) that you would want to treat as a reference for the end credits and then design your titler to match the font size relative to the image size. Keep in mind that a 4K render will likely allow a finer font size than would be ideal for an HD project (even more so for a 6K or 8K movie), and a consideration is whether the end credits are going to be a series of still credits or a credit roll.

Many tutorials by professionals about mastering music recommend using a favourite or 'target' song as a reference when mastering your own song - there's no reason that that approach can't be applied with other creative endeavours such as color grading, film/video editing style or even credits.

The OP PM'd me. Not sure why. The answer is: there is no answer. It is entirely up to you, and there are no set rules.

As I posted many years ago, there can be issues with smoothness of a title crawl, but if the titles are static, that problem doesn't exist. I don't use the modern versions of Vegas, so perhaps they finally fixed their crawl generator so you don't have to manually do the calculations to make sure that the top and bottom of the fonts line up on each frame with the pixel grid (rather than landing in between the pixels, thus forcing interpolation).