A 720×900 image is quite small for printing. You can keep the same aspect ratio when choosing a print size, but you cannot just convert pixels directly to millimetres and expect good quality.

At 500×625 mm, that file would be stretched a lot and is unlikely to look good, especially up close. Small prints may be acceptable, but large prints generally need much more resolution.

A good first step is to look for a larger version of the image. One answer suggested using a reverse image search such as TinEye to find higher-resolution copies. In the example given, a slightly larger version was found at 1024×1346 pixels, which would give you a better starting point than 720×900.

So: keep the aspect ratio, but aim for a much smaller print unless you can find a higher-resolution source. If possible, search for a larger original before ordering the print.