This is a nice function that I found a while back, but I can't remember exactly where
func adjustLabelFontSizeToFitRect(labelNode:SKLabelNode, rect:CGRect) {
// Determine the font scaling factor that should let the label text fit in the given rectangle.
let scalingFactor = min(rect.width / labelNode.frame.width, rect.height / labelNode.frame.height)
// Change the fontSize.
labelNode.fontSize *= scalingFactor
// Optionally move the SKLabelNode to the center of the rectangle.
labelNode.position = CGPoint(x: rect.midX, y: rect.midY - labelNode.frame.height / 2.0)
}
This adjusts the font size of the label to fit the width exactly, but you may want to change the function in order to add some extra padding on all sides.