STD Vistaprint Magnets

So I got the magnets, and want to send them out this week, but I need envelopes. My dad all different sizes at his work, but I dont knwo what size they are!!

I got them from Vistaprint. They aren't the small ones. They are large-no picture. Anyone know what size envelope? I dont have them here to measure, and I want to pick up the envelopes today.

Thanksssssssssss