Ow! Ow! Ow... You people all make my head hurt!

Victoria Lee has the answer!

When you import a background you have the option of setting your viewport setting to fit the background.

Unfortunately, this does not effect (affect?) your render dimensions.

You need to go to Render -> Render Dimensions

Then in Render Dimensions you can either manually key in your aspect ratio if you know it (I do all my renders at 1600x1200, for example) or if you don't choose -> Render to Exact Resolution, then check "constrain aspect ratio", then click "match document window".

Now, if you want you can increase or decrease the size of your render.  Changing one number will change the other one to the proper corrosponding number.

Make sure everything is set at "pixels".  The other measurements (inches, cms) are for print and will only confuse things if you just want to post ina gallery of work on your computer.

That said, choose a decent size, at least what your monitor display is set at, for example.

(HD Television is 1920 by 1080, but that would only print out at 6.4 inches by 3.6 inches at 300 dpi... a low quality magazine setting.)

(Don't bother changing from 72dpi unless you are going to print... even then I would just change that in Photoshop later... 72 dpi is dots per inch of all TVs and computer displays... when going to print it gets kind of complicated and that's a whole different discussion.  Others will chime in and try to tell you otherwise... ignore them.)

I hope this helps!