Hi Jeff,
Your document size isn't exactly important. vBulletin does not interpret the .psd extension. When an artist/coder designs a layout in Photoshop, the layout is imaged sliced and coded to "wrap" around the forum. vBulletin does not do this automatically, all of this work is completely manual.
What you can do, is design your template in photoshop and read some slicing and coding tutorials. Then once you have sliced your images, make a plain html mockup of your layout. Afterwards edit the css and templates in your admincp reflecting your html mockup.
To code a template you need to be familar with:
CSS (cascading style sheet)
HTML (hypertext markup language)
To learn to slice a photoshop layout you can visit
http://www.good-tutorials.com/ or
http://www.pixel2life.com/ for tutorials.
I hope this has helped you.