Same as any other app - create a new file 8.5x11 and paste your image into it. 1/2 will simply be blank. Now print.
Thanks! I know what you're saying, but it's an extra step I was hoping I wouldn't have to take. Aperture doesn't appear to have the ability to open a blank document and paste (and position) an image into it. It means opening a second application, doing what you suggest, then printing. I'm just being lazy.
I was hoping there would be a way a user could position an image on a sheet of paper when printing from within Aperture, but it appears Aperture will always force the image into the center of the page.
If nothing better comes along I'll do that.