The Zenith is the wide-spool version of the lightweight series. I don't have one but I think it's the same diameter as the Princess (3-½"). I use a St. Aiden 3-¾" on my lightest weight rods. It's typical of the size used on 8-9 weight single-hand rods. Despite its smaller diameter, the wide-spool Zenith has more line capacity than the St. Aiden but, as said above, its lacking the capacity for larger lines for two-hand rods. It's definitely too small for a 10-weight spey line.
If you want to use one of the Hardy lightweights for a 10-weight two-hander, find yourself a St. Andrew. It's essentially the same reel as a Marquis Salmon No. 2 -only sans the exposed rim.
The Lightweights and Marquis series are my favorite reels, whether it be the featherweight, LRH, Princess, St. Andrew or one of the Marquis models. They are all simply a joy to use in either series. I do like perfects a lot too, but something about the lightweight and Marquis models just feels right in my hand like no other reels.