Mongoose Select Populated Data's Size

In my blog model;

...
user: {
    type: mongoose.Types.ObjectId,
    ref: 'users',
    required: true,
},
...

And this is my user model;

follower: [
    {
      type: mongoose.Types.ObjectId,
      ref: 'users'
    }
],

So I'm gettting data like this;

const Post = await Blog.findOne({slug: post}).populate({
      path: 'user',
      select: 'name image count(follower)'
})

Of course count(follower) not working. I want to get followers count, how can I do that?