No easy answer, so much depends on the application. IF your boards are the same size, you can assume about thirds for thickness. It canít be stressed enough how important the shoulder is to stabilize the joint and prevent racking so donít skimp on the size of the shoulder thinking you are making a stronger joint by making the tenon as thick as possible.
The width should be as wide as possible, but not too wide or you have to deal with wood movement within the joint. In really wide joints it is customary to make double mortise and tenon joints.
The length depends on thickness of receiving part and if you want a through tenon or not. In a closed tenon you want as deep as you are comfortable cutting the mortise while making sure there is sufficient wood to keep the bottom of the tenon blowing out the back of your piece. Also, the more stress the joint takes the deeper/longer mortise and tenon will provide more strength so application is important.
In woodworking there is always more then one way to accomplish something.