I sell two poly practice chanters in two sizes. I personally like the McCallum or the RG Hardie. These chanters have the true pitches of the bagpipe chanter. You want to train your ear to hear the true bagpipe scale.
The length of the practice chanter should be from your mouth to the top of a table. You don't want to be holding the chanter in your hands while you are playing. If you are tall, you might want to buy the long poly practice chanter.