Guidance for AliExpress main images asks for 800 x 800 pixels or larger, with square as the shape every source accepts. This resizer exports a 1500 pixel square so your photo has room to spare.
We could not open AliExpress's own rule page, so confirm the numbers in your Seller Center. Your photo is processed in your browser and never uploaded.
What to keep off the image
A search excerpt of AliExpress's rules says to avoid borders, watermarks and picture collages, and recommends a white or solid background. The resizer changes size and shape only. It will not remove any of those for you, so start from a clean photo.