When copying files from one place to another, the operation can sometimes be interrupted mid-file (e.g. USB cable falls out, network connection drops, disk full, manual cancel). It is sometime useful to be able to resume such an operation (I often have this problem when copying music to my phone, but also with network transfers).

In such cases if I try to copy over the existing files, I get three options: overwrite, skip, or cancel. Generally, skip is fine, but if there was an operation that was killed mid-copy, then I can be left with some half-complete files. Overwrite fixes that problem, but then it wastes needless time, disk writes, and network bandwidth.

It would be nice to have an "Overwrite if different (size)" option, that would skip files that are the same size, or overwrite files that are different. This would be the best of both options.

It would not catch some edge cases where a file has changed, but the size is exactly the same, but I think those cases are probably extremely rare..