Find a minimum font size for a collection of strings (untested)

minFontSize.swift

func minimumFontSizeForTitles(titles: [String], boundingSize: CGSize) -> CGFloat {

    var minimumFontSize: CGFloat = 100
    let labelForSizing = UILabel(frame: CGRect(origin: CGPointZero, size: boundingSize))
    labelForSizing.font = UIFont.systemFontOfSize(minimumFontSize)
    labelForSizing.adjustsFontSizeToFitWidth = true

    for title in titles {
        labelForSizing.text = title
        minimumFontSize = min(minimumFontSize, labelForSizing.font.pointSize)
    }

    return minimumFontSize
}