The maximum Image size for images hosted on Linn Systems servers is 1800x1800 pixels. It is not possible to increase the image sizes stored on our servers. 

However, you can upload your images to an external hosting server and specify the links to these images in Extended Properties in My Inventory - IMAGE1, IMAGE2, ..., IMAGE12. This way, the images will remain hosted on the external server, and Linnworks will send links to these images to the channel when you create or update listings.

For more information on using images via Extended Properties, please refer to this documentation article.