Why 1500 × 600
A Notion cover spans the whole page, so it needs a wide image. 1500 × 600 pixels (5:2) is sharp on large screens and keeps the file light. Notion accepts WebP, so WebP is the default here.
Keep the middle important
Notion crops the cover differently on wide screens, narrow windows and phones, and you can reposition it after uploading. Put what matters in the middle band of the frame and leave the edges for background.
Small file, fast page
The cover loads every time the page opens. Watch the file size in the result panel and aim for a few hundred kilobytes; for a screenshot with lots of text, check it in Compare before you save.