To size your proxies correctly you will need a template to make it much easier.
You'll need to download Sribus for this one. Don't worry its free.
I use a PC with windows 8. I do not know if this works well on Mac.
Here is the Card Template. It wont show up on the Google Drive previewer but it will download what you need.
Here's what you should have after opening the file.
Now all you have to do is just drag your high resolution Magic: The Gathering Cards like this.
Then just fill up the page with everything you need and they should be ready to be perfect prints.