What is the leanest format for AVAudioRecorder?

Question

I would like to record voice using AVAudioRecorder, and it really doesn't have to be high quality at all as long as it is coherent. What is the leanest settings I should use to attain the smallest file sizes?

Right now, I use the following, but am not even sure if some of them are relevant to my using .aif files (IMA4).

        NSDictionary* settings = [NSDictionary dictionaryWithObjectsAndKeys:

            [NSNumber numberWithInt:kAudioFormatAppleIMA4], AVFormatIDKey,
            [NSNumber numberWithFloat:11025.0], AVSampleRateKey,
            [NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
            [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
            [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
            [NSNumber numberWithBool:NO], AVLinearPCMIsFloatKey,            
            nil];

Any advice on how to make my resulting files as small as possible? No holds barred on reducing the quality of the audio sample.