Why can I only set the longest edge?

Because it is what people actually mean.

Does resizing lose quality?

Scaling down is generally clean.

Pick how long the longest edge should be and everything scales to fit. The aspect ratio is kept, and nothing is ever enlarged.

Drop your images here

JPG, PNG, WebP, AVIF, GIF or BMP. Nothing is uploaded.

Longest edge

or exactlypx

The aspect ratio is kept, and nothing is ever enlarged.

Output formatQuality90%Visually identical to the original for most photos.

Why can I only set the longest edge?: Because it is what people actually mean.
Can I make an image bigger?: No, and that is deliberate.
Does resizing lose quality?: Scaling down is generally clean.
Are my images uploaded?: No.