I have allocated 25gb of free space on my disk and I was wondering if I was to select the automatic setting when installing fedora will it use the free space which is already on the disk. I'm trying to dual boot with Windows 8.1 and I also don't want to lose any of my files
The Fedora installer will not use space that is already in use by existing partitions.
If you select Automatic partitioning, it will try to use the free space on the disk.
Only if there is not enough free space on the disk, it will print a warning and offer you to delete some partitions, but you are not forced to do that.