The icon templates are only for applications icons (i.e. desktop shortcuts, program icons, favicon icons), not for logos.
If you want a logo of some specific dimensions just use any template (default is ok) and create the artwork in the dimensions you desire (using pixels as units).
Once you're ready, export as PNG using 90 dpi resolution (inkscape default) and that will result in a bitmap image with the exact dimensions you chosen inside inkscape.
Keep in mind that if you want pixel precision for your artwork is a good idea to activate the grid and change its distance to 1px in the document settings and turn on snapping. That will let you snap to exact pixel locations and avoid antialiasing problems in the exported bitmap (it's not a required step, but exported images look crisper when you use pixel precision).