When I had a full 34" scale 6 string bass, tuned in D standard tuning, as in 2 half steps bellow E standard tuning, that is D - G - C - F - A#- D# I used the 6th gauge .024 string from D'Addario's XL156 Bass VI set for the D# string, since those are longer than regular guitar strings, making it the perfect length, since the tuning key for the highest string on a 6 string bass usually is the one closest to the bridge.
To not make the guitar sized ball end of the string go through the string hole in the bridge I just threaded it through the cut off ball end of an old bass string.
Of course you would waste your money by buying a whole set of strings just to use 1 of them, but it works at least.
I think the gauges I used for all 6 strings was: .105 - .080 - .060 - .045 - .032 - .024, which was close to perfect balanced tension, and as I prefer a pretty low tension this might also be perfect for you, even if you tune in E standard tuning.