How large is the maximum Gig size for a Apple Photo Library?
There is no such maximum. Photos doesn't care about disk storage.
Here's the theory:
There is no such maximum. Disk space is simply that. It makes no difference to Photos if you have an image that is 30kb or 30MB. It just records the image in its database. The amount of disk space it uses is not really relevant.
Photos is designed to open Aperture Libraries, and they can have up to 1,000,000 items. So Photos can have up to I million items in its database.
So, in theory, as long as you have less than 1,000,000 items and have sufficient disk space, you can combine all your Libraries into one.
In practice however... very large libraries will be slow to search. How slow? Who knows? Depends on the machine you have and the amount of Ram you have on it, plus whatever else is running and the time of day and wind direction. Well maybe not the last two, but the point is that no one can tell you what the performance will be like with a very large library because there are too many variables that impact on performance.
Only way to find out: back everything up. Then try it. See how it goes. If it stinks, then you have your back ups to revert to and time to consider other options.