Which slide size in combination with which screenshot size or recording size?

Hello Patricia, 

Thanks for reaching out! 

We don't have a specific screen-size that we can recommend since a lot of factors can influence what the ideal resolution is for a project, but you can check out this article which explains how to get high quality video and images files for your course: 

• Storyline 360: Best Practices for High Quality Images and Videos

Additionally, having a large slide size and media files shouldn't affect the performance of your slides.

Hope this helps!