Disk Free Size

To know how much free disk you have available on your iPhone/iTouch just execute this simple code:

NSDictionary * fsAttributes = [ [NSFileManager defaultManager]
                    fileSystemAttributesAtPath:NSTemporaryDirectory()];
NSNumber * freeSize = [fsAttributes objectForKey:NSFileSystemFreeSize];

You can also use the same dictionary to know the total disk size by reading the NSFileSystemSize key.