Hi, I'll try to explain the main points about image size:

1. when you set the width (for instance "21" in your example) of an Image Target, the system will automatically compute the height, as it will use the aspect ratio of the image that you have uploaded (for instance, if you upload an image which is 1000 x 500 pixels, the aspect ratio will be 1000 / 500 = 2   => therefore your image height will be automatically computed by the Target Manager as "width / aspect_ratio = 21 / 2 = 10.5"  in your example)
2. The size of an Image Target is in 3D space; if your printed (paper) target is for instance 21 cm in width, then the "right" size if indeed 21 (width), if you imageine your 3D world measured in centimeters; however, if you assume everything is (for instance) in millimeters, then you should specify a size in millimeters, i.e. 210 instead of 21 (since 1 cm is 10 millimeters); actually, you must simply make sure that all your targets are defined consistently (i.e. if it is easier for your to imagine everything in centimeters, just set the size of all your targets according to the same unit of measure). 

Logo recognition is not brilliant - try other images to test.

You could try using mm rather than cm, but this probably will not make much difference.

N