What can I do about my odd sized heating vents?
The "odd size" is due to what was available at the time compared to what is available today. Your exact size may still be available, but not through regular retail outlets, they carry what's commonly demanded. A Heating &amp; Cooling Contractor or their Supply Store may be able to get new replacements readily. Also, scour the Internet, as many suppliers are now offering their catalogs to the world.

If you don't find yourself adjusting the Registers regularly, then you may have a system that was precisely balanced &amp; comfort is always fairly uniform. Changing the size of the Registers &amp; therefore their duct box wouldn't cause a problem or difference, just don't change any actual duct sizes.

Finally, why not repaint them or totally refinish them, if they're in good working order? A trip to an Automotive Store will have the Spray Paint Remover &amp; the new Spray Paint so you can fully restore them back to new in the color you want. Bent fins can be straightened, mangled screws can be replaced &amp; detached operation knobs or handles that are still around (in their absence a longer screw or bolt can suffice) can be drilled &amp; screwed for secure re-attachment.