There are 1024 bytes in a kilobyte and 1024 kilobytes in a megabyte, so...
NSLog(@"File size is : %.2f MB",(float)myData.length/1024.0f/1024.0f);
Mind you, this is a simplistic approach that couldn't really properly accommodate for byte sizes below 1,048,576 bytes or above 1,073,741,823 bytes. For a more complete solution that can handle varying file sizes, see: ObjC/Cocoa class for converting size to human-readable string?
Or for OS X 10.8+ and iOS 6+
NSLog(@"%@", [[NSByteCountFormatter new] stringFromByteCount:data.length]);
In Swift:
print(ByteCountFormatter().string(fromByteCount: Int64(data.count)))
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