Mazda5 19" alloy wheels would it fit? Do I need to calibrate the speedometer?

As long as the overall circumference of the tyre is the same as the original, then the speedo will read the same, but if there is significant difference then you need to change the speedo drive gear at the gearbox end. - usually remove the cable & withdraw & replace the drive mechanism. Contact a local Mazda specialist for the correct parts.