OK, I'm pretty happy with the guidance so far. I made a simple shape and learned a lot just from the process. But try as I might, I can't figure out how to resize something. On the workplane it's 12 mm which is what I want but the print comes out 36 mm. Since that's a 3x ratio I figure it's got something to do with that. But I don't know what.
Also, I am trying to make a taxicab rooflight in 1:48 scale. In tinkercad is there any way to take a basic box and turn it into a trapezoid shape? Or do I simply need to learn more? (probably option 2! Haha)