1920x1080
Roblox supports .png and .jpg formats for thumbnail images, although .png is recommended. “It is also recommended to use images with a resolution of 1920x1080 so that it displays nicely across all platforms (Xbox, Mobile, Computer).”
I figured that you might ask about the Icon too so…
512x512
Your icon must be a square, with maximum dimensions of 512x512 pixels. Your icon will be shrunk down and used at different resolutions throughout the site. An example of this is the Games page, which will display your icon at 128x128 pixels. We recommend you save your icon at multiple sizes to see how it looks.