How do I copy legal size documents

On an A4 scanner the process is a bit more complicated than on a sheet fed scanner.

You have to do two scans on these types of document.
First scan the top
Then scan the bottom
save each as different file names eg. legal1.jpg and legal2.jpg (assuming you save in jpg format)

Now with a suitable graphics program you splice the two pictures together carefully.

This may mean a re-scan if you got one of the scans crooked or badly wrong.

If you have a person that understands graphics packages they could show you quite easily

There are lots of different graphics programs and it depends on which you choose as to how easy or hard the joining of pictures is.