iOS Body-Mass-Index (BMI) app
You are doing double force-unwraps, twice. This is really just asking for trouble 

As others have pointed out in their comments, there are a few edge cases you should guard against. Good thing is you can do both at the same time.

Additionally, you should avoid using shortened names for your variables. In Swift, we usually type everything out.

I've updated and annotated your code below, take a look;

@IBAction func clickHereButton(_ sender: UIButton) {

    // Start by ensuring we have actual values entered by the user
    guard let name = nameText.text, 
          let sizeString = sizeText.text,
          let weightString = weightText.text else {
        return
    }

    // Then, ensure the values are valid Doubles
    guard let size = Double(sizeString),
          let weight = Double(weightString) else {
        return
    }

    // Finally, ensure that size &gt; 0
    guard size &gt; 0 else {
        return
    }

    // We now are 100% safe and can start calculating our BMI
    let bmi = weight / pow(size, 2)
    let bmiRounded = (round(bmi * 10) / 10)

    // Even though this var name is longer, it's clearer what it is.
    let alertViewController = UIAlertController(title: "BMI for \(name)", message: "Your Body-Mass-Index is \(bmiRounded)", preferredStyle: .alert)

    // You can use the `.init(_:)` shorthand when a specific type is required
    alertViewController.addAction(.init(title: "Okay", style: .default))
    self.present(alertViewController, animated: true, completion: nil)
}


The only other thing I would improve is to move your actual BMI calculations to a specific function that takes in the size and weight parameters.

For the rest, it's pretty much all good.