There’s a couple of reasons that the file size isn’t included as an option.

First, Lightroom gets the file size “on demand”, which means that for files that are missing or offline, such as on external disks or disconnected network volumes, the value simply isn’t available. This is probably not a huge deal for most people, but it would make the search unreliable in some cases.

The bigger reason is that it simply doesn’t add anything to the search.

All the other comparisons are for actual image data; even if the file is stored as separate raw, jpg and DNG copies, that data stays the same. File size is completely separate from file contents. Two random photos might well have the same file size, and two identical photos may have different file sizes. Lightroom is able to write data back to most kinds of file, and this will change the file size for two copies of a single photo too easily for a file size comparison to be of much use.

There’s a collection created during the search which will tell you which of a set of matches is largest and smallest.