How do I call the size property of a part to change it in the script?

I'm trying to create a drawbridge-type thingy, and t``he way I want it to work I know that I need to change the length property of the part, how would I call that property in the script? I know what the script should look like, I just don't know how I would call that specific property of the part from the part.

The mapping to the part is Game.Workspace.Model.drawBridgeArm