GUIText.fontSize  

var fontSize : int

Description

The font size to use (for dynamic fonts)

If this is set to a non-zero value, the font size specified in the font importer is overriden with a custom size. This is only supported for fonts set to use dynamic font rendering. Other fonts will always use the default font size.

guiText.fontSize = 12;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        guiText.fontSize = 12;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        guiText.fontSize = 12