How much disk space should I allocate to Bootcamp

The issue depends on how much disk space you will need for each operating system. For that, I wonder what applications you intend to run on each side. As a baseline, expect the operating system and basic applications for each side to require 16gb each. In some cases, the data will not need 16, but it gives you an assumption you can use for the amount of free space that will be available, especially considering that as the machine is used, system files build up and will require more. Therefore, you need to balance the free space based on the applications you want to run. If you are using the computer just for word processing, e-mail, web browsing, and the like, then disk space is not that important. If you intend to store a music library, sync an iOS device for backups or even just to keep the data connected, or maintain any project that will require more disk, make sure that the side you plan to do this on has enough free space. With a 128 GB disk, I would expect that large projects might require external storage.
On the whole, I would say that a balanced 64 GB for each partition should function well. A shift up to 80 GB to one side or the other is doable. There will be no system problems. However, depending on the data you intend to store, the side with the 48 GB may feel cramped. Because it is not easy to change the size of the partition after the installation, it can be useful to let us know if you intend to use projects that take disk resources. These include anything with music, video, or many photos, backups for other devices, large applications, computer programming tools or projects, using the device in multiple languages, and virtual machines. Let me know and I'll do my best to better tailor the information for your situation.

Hey there!
In try balancing out your storaga evenly depending on which side you'll use. So if your Mac has say 24GB, I'd give 12 to bootcamp and 12 to mcOS, like the commenter above said, depending on what you'll be using on either side, and system files could probably make things differ.

As others have said, it depends in part what you'll be using bootcamp for. However, also take into account your computer's disk size. If its a large size, e.g, 500gb or 1tb, i would say half and half just to be on the safe side. You can still do half and half on smaller disk sizes... but you'll probably need storage devices handy. If you'll be using the mac more, then more space to the mac, same for windows, but if you'll be using both constantly... i would say half and half. Then again, there is the option of a vertual machine lol. good luck