how do you resize a model, for example make a BOP bigger
Open up the .nif file in a 3d modeller, resize as you would any model. One example, in Milkshape, is the assimsoft scaler tool that resizes it according to a length in metres, which it then converts to an appropriate scale. I can't say it is perfectly accurate, but the more mods are scaled with it, the more are scaled properly to my mods...
But you'd have to go into mod editor and then realign all the hardpoints afterwards! Just alter the xyzs by the same scale as the model resize, and alter the size of some hardpoints themselves if you've resized the BOP considerably. There aren't too many in a BOP so it won't be as arduous as it sounds!
Is there any other way to resize, like using the .PY file to change some numbers?
nope not using the .py file, only thing that can change is the mass and rotation speed.