Is it possible to see the size of an image in millimeters in Light Room? I can see the image size in pixels in the meta Data so is it possible to change this? I can obviously move the image to Photoshop to see image size.
There is no point until you export the photo, as it all depends on the resolution you export at to give the size. When you specify the size in the export dialog LR will calculate the resolution needed to match.
Digital photos do not have a length in millimeters or inches. They are only measured in pixels. So Lightroom does not report millimeters or inches.
Printed photos can be measured in millimeters or inches.
In Photoshop, the inches or millimeters shown is an assumption that you are going to print at a specific ppi, when in fact there are an large number of PPI values you could print at, so even the numbers shown by Photoshop are meaningless, you can always print the photo at a different size.