how do I find image file size in lightroom

When you send a raw file from Lightroom to Photoshop, it will first open silently in the Camera raw plugin, where the adjustments from Lightroom are applied.

Then a new file is created, rendered from the raw file, and this is the file that opens in Photoshop.

This new file is not a raw file, and until you save it, it's just a generic RGB file, and the file size is calculated using pixel dimensions and bit depth. This has nothing to do with the file size of the raw file, which has not changed at all, even if you cropped it in Lightroom.

Raw files are 12 or 14-bit one channel compressed grayscale files, whereas RGB files are 8 or 16-bit three channel files (compressed or uncompressed), so file sizes will vary a great deal.

A raw file from my Nikon Z7 is around 40 - 50 MB. The file size varies, because the degree of compression varies with image content. Sharp detail and noise is hard to compress, whereas flat, smooth areas are easier to compress.

An 8-bit Tiff rendered from a raw file from this camera will be 130 MB, in 16-bit it will be 260 MB.

Saving with compression, it will be somewhat smaller, depending on image content and the compression method used.

To actually crop the image, you have to export it.

Lightroom edits are non-destructive, and the edits are applied to a new file when you export, the original is untouched.

The image you see on screen is a preview, not the actual original.

Lightroom only knows the file size of the original, and cannot possibly know the file size of the exported file, cropped or not.

The file size of the exported file depends on a number of factors – like file type, bit depth, quality setting for jpg, as well as image content when using compression.

But you can see the cropped dimensions in pixels, in the field below Dimensions.

If you want the image size in terms of Pixels, it can be found in the Metadata on the right side Metadata Panel.  If you want the size in megabyte storage size on your hard drive, use you system Browser, Finder on a Mac, Windows Explorer on a PC.