How can I adjust the size of the OSXRESERVED partition that Boot Camp Assistant uses?
Apple provides instructions for installing Windows 10. See Install Windows on your Mac with Boot Camp These instructions provide a link to the Microsoft Windows 10 download site.

The October 2018 ISO (1809) has one file that exceeds the maximum file size for a FAT32 formatted partition. This can cause problems when the Boot Camp Assistant instructs a user to insert a USB flash drive which is FAT32 formatted.

Your Mac model is newer so you should not have this problem, since the OSXRESERVED partition is formatted ExFAT. For other users with old Macs, one possible solution, if problems occur, would be to use the April 2018 ISO (1803).

Is it possible to either:


A) resize the OSXRESERVED partition (taking some space from either the BOOTCAMP partition or the APFS container partition?)

It has been awhile since I ran the Boot Camp Assistant on a newer Mac. From what I remember, there was no way to stop the installation and customize the partitioning. You may be able customize by not using the Boot Camp Assistant. For the older Macs, the steps for installing were outlined in the question How to install Windows 10 into a 2013 iMac without using the Boot Camp Assistant, USB flash drive or third party tools? The answer is a bit outdated. I could update the answer if you post the output from diskutil list. I will admit that I am not sure if you would be better using the unattended install or not. In other words, I am not sure how exactly the OSXRESERVED partition is removed.
or B) tweak a 'OSXRESERVED partition size' setting in macOS that is used by Boot Camp Assistant and run it again?

There is nothing the the Boot Camp instructions for changing this partition size. The command defaults read com.apple.bootcampassistant does not show any useful defaults that could be changed.


Another problem to look out for is the use of the Windows Recovery Environment partition. In some cases, the partition is not allocated enough space. This can result in an empty partition. The files that would have ended up in this partition are being placed in the Windows partition.

After Windows is installed, you can verify the location of the Windows RE files by entering the command given below in an Administrator Command Prompt window.

reagents /info