The Chrome Web Store screenshot size
Chrome Web Store screenshots are 1280 x 800 pixels (preferred) or 640 x 400, as JPEG or 24-bit PNG without transparency. Your listing needs at least one and can show several.
How to make a good Chrome Web Store screenshot
Show the extension working on a real page, with a short caption. Use 1280 x 800 so the screenshot stays sharp on large displays.
Questions
What size is a Chrome Web Store screenshot?
Chrome Web Store screenshots are 1280 x 800 pixels (preferred) or 640 x 400, as JPEG or 24-bit PNG without transparency.
How many Chrome Web Store screenshots do I need?
At least one; listings can show several. Use them to show the extension's main features.
Is my image uploaded anywhere?
No. The image is read and resized in your browser; nothing is sent to a server.
How does the resize work?
It scales the image to cover the target size and crops the overflow evenly from the edges, so nothing is stretched.