What is the maximum texture size for a 3D model & how can i reduce the size of my textures?

The recommended maximum size for your upload is 50MB, and that includes the texture size. So you should try to reduce your texture size appropriately for your 3D file size, so that the whole package fits the 50MB soft limit. Check out this article, in which we check the impact of a lower texture resolution on the final print, and give you advice on the best compression to optimize the 3D printed texture resolution.