There is no such way to control the size of an image while sharing on Lightroom Mobile.
It will display you the same image preview that you have on your desktop. Image preview does not take much space on your mobile device as well as its not an actual image.
Thanks Sarika. Indeed, it doesn't weight much, but it takes space of a screen. It's a pity there is no way to change image size.
THanks again for you answer,