When converting a photo to a pattern we use GIMP to remove background, highlight features, and Filter/Artistic/Photocopy. Then save and load into Inkscape to Path/Trace Bitmap. This makes a nice SVG (scalable vector graphic) version of the image. The SVG can be resized at this point in Inkscape and will retain the smoothness of the image. So this is the best place to set the size of the image (before returning to GIMP - if you increase the size in GIMP you will see the jaggies showing up).
The top toolbar in Inkscape has X, Y, W, lock, H. If you close the lock then the W/H proportions will be maintained. Set W to your desired width (if you are making an 8"Wx10"H portrait, maybe set W to 7" to keep some border in your cutting). BUT now you SVG spills outside the boundaries of the Inkscape page. If you save it then load into GIMP, you'll only see the part of the SVG within the page boundaries.
To fix the Inkscape boundaries, go to File/Document Properties and click on the '+' next to "Resize Page to Content". The window expands, and at the bottom is a button "Resize page to drawing or selection". This will expand or contract the Inkscape page boundaries so they just surround your SVG. Then save the SVG and open in GIMP to edit for floaters and add bridges. I usually save the final version as a GIF rather than jpg (it's smaller and it flags it as the pattern).
