Yep, size D is the closest you'll get. Your 3.25 is an 0 in string hooks, Size wise, it depends on the pattern how critical the size of the needle is. I usually do OK with a little bigger hook because I tend to crochet tight.
I made this basket weave dishcloth from a link in the 50-dishcloths thread. It called for 4/2 but I used an 8/4 rug warp. It was fun to make, but the instructions get a little iffy when it comes to the edging, and you have to do a little adjusting. http://www.angelfire.com/oh/crochethook/patts/dish.html
Hi Meg, thanks for the response. Mostly what I was thinking about is that I have a lot of 8/4 rug warp in all different colors. I like to do doilies in #10 and wondered if they'd work up the same in rug warp. I never thought about putting some of the #10 in my balance to compare, I think I'll give that a try, along with the ruler wrap and see what it looks like.