Alright, first things first. Make sure you've got Node.js and npm all set up on your system. Then, head over to your project directory and fire up Firebase with this command:

firebase init

When the CLI asks, "Which Firebase CLI features do you want to set up for this folder?", go ahead and pick 'Functions'. Next, choose the Firebase project you created earlier in the Firebase console for 'Select a default Firebase project for this directory'.

Now, let's get those necessary modules installed. Navigate to the functions directory in your project and run these commands:

npm install --save @google-cloud/storage
npm install --save firebase-admin

Sharp is a fantastic module for handling images in Node.js. You can install it with this command:

npm install --save sharp

Next up, create an index.js file in the functions directory and import the necessary modules:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const gcs = require('@google-cloud/storage')();
const sharp = require('sharp');

Now, let's write a Cloud Function that triggers every time a new file is uploaded to a specific storage bucket:

exports.onFileUpload = functions.storage.bucket('my_custom_bucket').object().onFinalize((object) => {
...
});

The 'object' parameter here is a reference to the file that was just uploaded.

Time to resize that image! Inside the onFinalize function, add this code:

const filePath = object.name;
const bucket = gcs.bucket(object.bucket);
const fileName = filePath.split('/').pop();
const tempFilePath = `/tmp/${fileName}`;
const metadata = object.metadata;

return bucket.file(filePath).download({
    destination: tempFilePath,
})
.then(() => {
    return sharp(tempFilePath)
        .resize(800, 800)
        .toFile(`${tempFilePath}_resized`);
})
.then(() => {
    return bucket.upload(`${tempFilePath}_resized`, {
        destination: `resized_images/${fileName}`,
        metadata: metadata,
    });
});

This code downloads the newly uploaded file to a temporary location, resizes it, and then re-uploads it to the 'resized_images' directory in the same storage bucket. Pretty neat, right?

Finally, let's get this Cloud Function live. Deploy it with this command:

firebase deploy --only functions

Once this command finishes, your Firebase Cloud Function is up and running. Now, whenever a new file is uploaded to the storage bucket 'my_custom_bucket', it will automatically get resized to 800x800 pixels and re-uploaded into the 'resized_images' directory.