You can set the max size for a certain design in the DesignCreation file. For different sizes at the same time you can use different max sizes for defense and attack ships. Those will not share the same fleet, though. For different ship sizes in the same fleet you can use the boarding ship (they get included into attack fleets quite well). The boarding ship doesn't actually needs to have marines, you can make it any design you like and set the default strategy to optimum range (or whatever).
Hope that helps
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Rollo