I would like to upload a photo from LR mobile on my ipad to Pictorem but my file is too big. I would like to reduce MB from 51MB to 3Mb without losing pixels. The pixels in the original is 6000 x 7000 I don't want to lose that.

The only way to do this is to choose Export as, choose jpg as file type, select Largest available dimensions, exclude all metadata in More options and then experiment with the Image Quality settings. Of course you want high quality too, but high quality and small size don’t go together. So you will have to lower the quality until you get to only 3MB… Why do you want only 3MB anyway? Is that the maximum size Pictorem allows you to upload? That would be odd for such a service.