For upscale/downscale brainstorms, I usually make a list of the relevant dimensions in excel and multiply the column by the scale factor.
I have heard of OpenRocket's scale feature, but I've never remembered to use it.
If you generate your nosecone, any body tube/couplers will be scaled to its diameter. Same for tubes after a transition. When you choose your motor mount size, centering rings will automatically conform to the space between airframe and motor tube. That just leaves the fins.