hey i have a file .xy extension , how to open it ? notepas++ says its too big , i am trying to edit the source code?

First off, that's not a ".xy extension". The true extension comes after .xy and is not shown by Windows if it's a type registered to the system or an installed application.

If you want to see the true extension, go to Tools>Folder options... from any Explorer window and click on the "View" tab. Scroll down and uncheck the "Hide extensions for known file types" box. I pretty always do this on every Windows login I use.

That display says the file's extension is registered to WinRAR, and that's what you should use to open the file. It's probably a .zip or .rar extension and, If so, what you have is a compressed archive that needs to be decompressed into normal files (usually a whole subdirectory of them) before you can use normal applications to process them.

That file is 744 Megabytes in size. I doubt it's supposed to be opened in an editor directly.

According to Google, .xy is the extension used by XYWrite, a really old word processor. Can you confirm the actual format with the owner?