Is there a way to do that without uploading the pictures again, it's about 8GB of data.

IGraham wrote:

is there a easy way for the admin to see what the uploaded photo dimension and size (mb) are without having to go into the uploaded file structure to look at each individually

well you're supposed to know the dim and size, because you have the picture on your computer!
the only way on Piwigo, from the adminsitration part, is to go the edition page of a picture, where your will see something like :
•1024×768 pixels, 0.57MB

IGraham wrote:

having found a photo i wanted to resize is it best to reupload through piwigo or reupload direct to the file structure, does it make a difference ?

'reupload through piwigo' using the plugin which allows that or reuplaod as a new picture?

Generally the best is to think before uplaoding, and to set an upload max size, on the Picture size page, in the Options