To free up space in your EFI partition, you would have to first mount it. This can be done using the command given below. Enter the command in a Terminal application windows.
diskutil mount /dev/disk0s1
Below is an image of the entire contents of my EFI partition. I assume you can delete any folder and/or file other than what is shown below. (Actually, I am not even sure the
Firmware.scap file is needed. I do not know what it is for.)
Click on image for a better view.
When finished, you may need to empty the trash. Finally, enter the command below to unmount the EFI partition.
diskutuil unmount /dev/disk0s1
If you are doing BIOS/MBR install, then what is posted below applies
From reading other posts, I thought 2015 Macs can no longer be installed in BIOS/MBR mode. So this would apply to older Macs.
To change the system partition to partition 4, you need to run the following commands in a Terminal Application window. You need to do this before attempting to install Windows.
INPUT=$(printf "f 4\nw\ny\nq")
sudo fdisk -e /dev/disk0 <<<"$INPUT"
This will require your login password. This is normal.