File size can be only viewed in Metadata panel. It would be great if "File Size" could be included in Library View Options
As a partial workaround, you could use Megapixels instead (which is roughly proportional to file size).
This is a user-to-user forum that Adobe doesn't actively monitor (much). You might repost your request in the official Adobe feedback forum: Recently active topics in Photoshop Family about Photoshop Lightroom.
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