Barnbum,
I have been learning how to machine quilt, just like you are starting, and I found that 18" square was a better size, purely because before you know it you have got into the rhythm and you have filled up the practice piece, so I made mine bigger, another tip, when you have filled it with one colour of thread, change your top thread and go over all what you have already done - you can do this several times.
I made doggy bed covers with my practice pieces heres one...excuse the state of it, I just whipped it off Sashas bed!! :D :shock: