That size container should be more than enough! I have somewhat large rose-scented one that I just chopped back for cuttings last weekend and it is in an 8" pot that is probably about that deep. I recently repotted a rose-nutmeg one from the little 3" pot that it was in to a decorative 6".

As you probably know, these can get real leggy (I had some stems that were going on 3ft long on the variegated rose-scented...lol), so doing what you suggest shouldn't be a problem. Just make sure that you have a good soil mix in there that will dry fairly quickly since the scented (and regular) pelargoniums like drier conditions.

If you are worried about them intermixing roots, you could actually pot them in their "overwintering" pot and sink that in the windowbox (where the windowbox has soil in it), and cover all the sunk pots with some decorative mulch or equivalent to hide their rims. Make sure that the pots have drainage too. That way the plants can grow lushly but will still be separated. If they get potbound, the roots can grow into the surrounding soil of the big windowbox via the drain holes, AND you can more easily pull them up in fall to bring inside. That should be neat to do.

oh wow what a neat idea!! I didn't even think about putting them in there in their own pots! That is exactly what I am going to do. They will be in good sun while it is warm and only actually have to be in the house about 3-4 months out of the year so maybe mine won't get too leggy!! I hope! I can't wait to do this!! I would love to get some more suggestions!! Thanks!

Carly