The first thing is that I wouldn't ship a package in a mailer that small. Canada Post has minimum package requirements. You're not saving any money on shipping rates by shipping it that small compared to using a 6"x4" bubble mailer. Granted, I am not saying they will reject your package if you drop it in a mail box. It may still get delivered, but it's pointless to ship something in packaging that small.
My advice would be to take the package as you have it now, and place it in a paper or bubble mailer. Then place the label on the mailer.
To answer your actual question, when you print something out on a computer there should be an option called something like "scale" that allows you to alter a percentage. The default percentage is 100. You can lower the percentage down to get it to print smaller.
