Another option, if you have the kit in hand, is to use a good set of calipers to measure the actual hardware where it meets the wood, which will often give you a more accurate target than going by the bushings (whether their actual or theoretical dimensions). Of course, sometimes that works better than others. On a kit like the Gatsby where the wood meets a bead on the hardware, it can be hard to tell just where to measure... but, by the same token, that sort of hardware also tends to allow for a wide range of sizes without looking/feeling wrong.
If, on the other hand, you're trying to look up bushing sizes to see which kits will work with the bushings you already have, that's no help at all, and you're probably stuck falling back on Curly's suggestion of hitting up the vendor for more data.