Personally, I would go with a 5" vise. The main thing I look for is the distance between the center T-slot and the rear of the table. The clamping ears of the vise sit over the center slot and you don't want too much of the rear of the vise sticking out toward the column; this interferes with Y-travel. I also don't like having to peek around the handle end of the vise to see the indexing wheel for Y-travel; the less sticking out, the better.
I think a 6" vise fits fine on a 9" wide table. My mill has an 8" wide table and a 4" Kurt vise fits perfectly. I don't do really big work so the 4" is fine for me. Whichever vise you buy, buy a really good one.
If you need to work on something larger than the vice can comfortably hold, you can just remove the vice and attach the part directly to the table. Also Be sure to consider the weight. while I keep my 5" Kurt vice on the table most of time, I occasionally need to move it off. I can just manage this without help for the 5". If I had a bigger vice, I would need help.
I have an 8” x 36” table and am quite happy with the Kurt DX6 I bought for it. The worst part is there can be interference with the big speed handle I bought from Grizzly, but I plan to make a shorter one.