So, to sum up how to change ship scale:
1. Select the ship that you want to import onto another size class.
2. Group all the parts together and save as a new "custom" part.
3. Start a new hull, and then import that one new "custom" part onto your new size hull. Congrats, you just have changed ship size class.
Some things to remember are that all grouped parts should be attached to each other in the original design. Its best to only group basic parts as a new grouped custom part, and not use certain already grouped parts to make a new grouped part. The reason is that changing the overall size of the new custom part will not change the size of the small grouped part, and one will have to rescale by hand.