File > Print > Properties
Select the down arrow next to Paper and choose Other Size > Advanced Paper Size
And assuming the document is actually Legal and all is right in the document, you should match this screenshot exactly
If that still prints wrong, the application is the cause and you would need to look there. You can't for instance just increase a Letter job to Legal because it won't magnify it at all, because it already goes to the 8.5 inch margin and it can't increase in only one aspect, so it will cover the center of the legal job exactly the size it would on letter top to bottom. You can see the margin in the preview in Acrobat/Reader
Other programs obviously have different displays
I have the Xerox color cube 8580. I need to print a LEGAL sheet. I change the page layout to legal on the printer, change it in the program and when it prints it starts about 2 plus inches down from the top of the page cutting off the same from the bottom.
Any suggestions?