The rule I follow is that the size of the mortise should be 1/3 the width of the wood and the mortise should be centered in the wood. This establishes the bit size. The length of the tenon is determined by what you are trying to join.
Youtube has some excellent videos by Halfinchshy and New Brit Workshop. You will save yourself a lot of grief watching these videos followed by practicing on scrap.
The Domino is a wonderful tool, but it requires good technique to ge good results.