The recommended thumbnail size for things like manga series and profile pictures is as follows:
Size: 1200 x 630 px
File Size Limit: 32 MB
File Types: JPG, PNG
* The image will be displayed with the same ratio on both the desktop and mobile version of pixiv.
It's possible that a portion of the cover image may be cropped, resulting in the edges being removed. Please refrain from setting an image in violation of our Guidelines.