EditorUtility.FormatBytes

public static string FormatBytes (int bytes);

Description

Returns a text for a number of bytes.

using UnityEngine;
using UnityEditor;

public class FormatBytesExample : MonoBehaviour
{
    void Start()
    {
        // prints "100 bytes"
        print(EditorUtility.FormatBytes(100));

        // prints "2.0 KB"
        print(EditorUtility.FormatBytes(2048));
    }
}