Cannot decrease Windows partition size to less than 1058 GB

OS: macOS Sierra, Version 10.12.6

Model: iMac (27-inch, Late 2012)

Processor: 3.4 GHz Intel Core i7

Memory: 16 GB 1600 MHz DDR3

Startup Disk Storage: 3.11 TB

Graphics: NVIDIA GeForce GTX 680MX 2048 MB

Boot Camp Assistant: Version 6.1.0 (6050.60.1)

I have already created an install disk and downloaded the latest Windows support software, but I have a problem when I try to add a Windows partition to the startup disk. The slider bar does not allow me to make the partition any smaller than 1058 GB, which I know for a fact is WAY more than you need for a partition. Also, I only have about 313 GB available on my startup disk, so I couldn't set up a partition with a size this big, anyway, so why is it asking me to do so? I know the problem isn't in the ISO image or the version of Windows I'm trying to install; I tried two different ISO files (one for Windows 8.1, and one for Windows 10) and they both had this problem. In the image below, the divider on the slider bar is dragged as far to the right as possible.

I believe the problem is in Boot Camp Assistant itself, because this slider bar seems really, really broken, and not just because of the 1058 GB minimum. For example, when you drag the divider, the partition sizes don't increase/decrease by increments of 1 GB. Rather, the size will apparently not change, then suddenly jump by a large amount (e.g. from 1058 GB to 3106 GB, or from 3106 GB to 5153 GB, as shown in the next four images. At this point, the partition size would be larger than the amount of storage in the entire startup disk).

I can even drag the divider of the slider bar so far to the left that it leaves the window.

With all of this, I think it's safe to say that the application itself is pretty broken. Is there any way to uninstall, then reinstall Boot Camp Assistant without completely reinstalling macOS? Or is there another solution that you guys suggest? I included some tech specs at the top if any of those help.

Final note: I haven't actually tried to install the partition, because if it was successful, and the size of the partition was as big as Boot Camp Assistant says it is, there would obviously be no room for over 500 GB of the files in the Mac partition.