On 32-bit systems, a RAM disk can be up to approximately 3.5 GB.
On 64-bit systems, a RAM disk can be of any size. Within the limits of available RAM, of course.
Also, just for your information, some years ago a few users reported that they were unable to create a boot-time RAM disk larger than 150 GB. The cause of that could not be determined, but those users successfully dealt with the problem by creating several smaller RAM disks. In either case, the 128 GB disk you are planning to create should not be affected by that issue even if it still exists in the latest version of the RAM Disk.