You can resize props by making them a prop_dynamic and setting the "Model Scale" keyvalue, though do note that this will not change the collision model for the prop. If you want to change the scale of a fully static prop, or need to change the collision, you can decompile the prop using something like Crowbar, adding or changing a $scale line in the .qc file, and then recompiling with that .qc file (and also changing the prop's name so as not to create any conflicts).
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