I've tried using Inkscape .48 to convert a SVG file to a .png file to use on CafePress poster and when I submit the .png to CafePress, it gets shrunk.

The canvas size in Inkscape is: 22.85"W x 34.80"H
Layer size in Inkscape is: 22.801"W x 34.591"H
Poster size on CafePress should be 23"W x 35"H

Inkscape creates a 1.2MB PNG file and a poster size on CafePress of just 25"H and a lot of white space.

How can I fix this?

Hi,
I have a couple of questions, a couple of comments, and potentially a solution, although I'm not sure if I understand the problem correctly.
When you say

Inkscape creates a 1.2MB PNG file and a poster size on CafePress of just 25"H and a lot of white space

Do you mean that the poster is 25" high and proporionally smaller in width, with a lot of white space where the image should be? Or do you mean that it's 25" tall and really skinny, so that the original proportion of the dimensions is lost?

As long as the image is staying in the same proportions (even though smaller), I'm guessing that CafePress' default DPI is different from Inkscape's, which is 90 DPI. The same thing happens with Adobe Illustrator, and we know that's attributable to different default resolutions.

Is CafePress a web-based service, or do you have some software? Well, either way, if you can find out what the default resolution is, you can adjust the DPI in the Export Bitmap dialog to match theirs. Or if you can't find out, you can just use trial and error, until the result is the proper size.

But if you mean that the proportions are different, that could be a whole different problem.

You're right. It was proportionality smaller and I changed the DPI and it works. Thanks!