How to determine the size of a new file after cropping in Photos?

Sussexminman wrote: …I then save my final image in whatever folder I choose.

You are using the Photos app, right? Do you mean that you "save" the picture in Photos? If so, then it has no size, because there is no file-- just the information to make one.

Do you Export the picture to a Finder folder? In that case, Finder being a file manager, it will give the size of the file you chose to make.

When you use File>Export>Export nn Photos, you get this dialog:

Here you can choose the format, the quality, and the size of the exported picture, and all of those choices will influence the size of the file. As I said, the file size, even for the same quality and image size, will be different if you choose HEIC or jpg or TIFF or png. The file has no size until those choices are made.

The Quality is about information loss in the compression used to construct the file. The Size is the number of pixels, and that influences how large you can print the image. The full size number of pixels is given in the info window in Photos. "Custom" is offered to let you choose the number of pixels.

You can try out the different choices and see what you get and how the choices affect the size and quality of the image.

Sussexminman wrote: … same as the original file size (before cropping) which isn't a lot of help.

It's not clear what you want to help? Cropping isn't normally used to make the file size smaller-- it makes the image size smaller.

In fact, when you edit or crop an image in Photos, there is no file, so there is no file size. Photos is a non-destructive editor. That means that the original file is untouched, so the file size remains the same. When you crop a picture, Photos remembers what you did, and when you look at that picture in the future, Photos will use the original and it's recording of what you did to reconstruct the edited picture on the fly. That's what you can always "revert to original." Some temporary files are created, but there is no file that is the edited picture until you save the edited version as a new file.

The size of the file you saved, then, depends on choices you make about the quality and type of the compression that you use. For instance, if you choose the same quality, an HEIC file will be smaller than a jpg file, but they will look the same.

1 - Bring up the Info pane in Photos for the original file and record the pixel dimensions.

2 - enter the Edit mode and crop the photo to your specifications.

3 - Click done and go back to the library and select the edited photo.

4 - bring up the Info pane for the edited photo and compare the pixel dimensions to what you recorded for the original.

I got this for the original:

and this for the edited version: