what I did is something like this:

swift 5.x

let myString = "Some text is just here..."
let size: CGSize = myString.size(withAttributes: [.font: UIFont.systemFont(ofSize: 14)])

swift 4.x

let myString = "Some text is just here..."
let size: CGSize = myString.size(withAttributes: [NSAttributedStringKey.font: UIFont.systemFont(ofSize: 14)])

swift 3

var originalString: String = "Some text is just here..."
let myString: NSString = originalString as NSString
let size: CGSize = myString.size(attributes: [NSFontAttributeName: UIFont.systemFont(ofSize: 14.0)])

swift 2.x

var originalString: String = "Some text is just here..."
let myString: NSString = originalString as NSString
let size: CGSize = myString.sizeWithAttributes([NSFontAttributeName: UIFont.systemFontOfSize(14.0)])