I'm not sure about bokken but the usual standard for iaito is handle=1/3 of blade.
I would agree that just sliding the tsuba up isn't the way to go. If you want extra handle either have one custom made or get a bokken that's bigger all around.
Personaly, I like a bokken with a little bigger handle.
Kingfisher Woodworks was able to add 2" to the handle of their aiki bokken for an additional $15. I also have a white oak one that I think came from The Kiyota Co. (Google their name for contact info as they don't have a website). The whole bokken was bigger than standard. I actually ended up having to shorten the blade on that one by a little as it was just too big for our low ceilinged dojo.
Bronson