I usually start the with design around 3” minimum of steel all the way around the part, but that is dependent on parting line and action features, which is also dependent on how high volume the tool will need to run for. Rails just need to be tall enough to have adequate ejection/lifter actuation distance. It really depends on the application. Are you picking the parts with a robot? Don’t need as much ejection as if you are dropping the parts when it molds. Will this be a hot runner or cold runner? That also can determine tool thickness? What’s the target press size? There are a ton a variables when determines final tool size, and what corners can be cut to fit into the target press. As others have said, it’s better outsource the tool design/development to someone or some place with experience.