Sizemod is a percentage of the hull's capacity that is added to the components size. No hulls will ever get the component at its base size (that would require a hull with 0 capacity...).

Defenses are rolled just like attacks. So for example if you have 5 armor, your ship would have a chance of stopping 0 to 5 mass driver damage each combat round. Each defense type is paired to an attack type, and is only worth it's square root rounded down against other types (so 5 armor would roll 0-2 vs energy or missiles).

Thank you very much!