Can somebody explain why when I choose an image file path in my RAM disk settings, the size and file system fields become greyed out?
What's the correct way to set the size and file system for both the RAM disk and its image file?
When you are creating a volatile RAM disk (i.e. the one that will be used without an image file and will have all its contents lost every time you reboot or power off your computer), you should leave the image file name field empty in its settings, and instead specify its size and the file system. The disk will be created based on the set size and file system.
When you are creating a persistent RAM disk (i.e. the one that will be used with an image file and have its contents derived from [and optionally saved to] an image file), you will need to create the image file first, specifying its size and the file system, and then specify the path to that image in the disk settings. The disk will be created based on the size and file system of its image; that's is why those fields become disabled in the disk settings in this case.
In both cases, it is important to remember that the size of the RAM disk or the image should not exceed the amount of free memory you have (i.e. the amount of memory you have good grounds to believe your system never uses).