here are only three valid heights for UIPickerView (162.0, 180.0 and 216.0). 
You can use the CGAffineTransformMakeTranslation and CGAffineTransformMakeScale functions to properly fit the picker to your convenience. 
Example: 
CGAffineTransform t0 = CGAffineTransformMakeTranslation (0, pickerview.bounds.size.height/2);
CGAffineTransform s0 = CGAffineTransformMakeScale       (1.0, 0.5);
CGAffineTransform t1 = CGAffineTransformMakeTranslation (0, -pickerview.bounds.size.height/2);
pickerview.transform = CGAffineTransformConcat          (t0, CGAffineTransformConcat(s0, t1));
The above code change the height of picker view to half and re-position it to the exact (Left-x1, Top-y1) position.
Refer more here.