[JsonIgnore]
            public float MaxFileSizeInBytes
            {
                get
                {
                    return MaxSize * 1024 * 1000;                                      // <<<<  1024 bit * 1000, and not * 1024 ! :)
                }
            }​

Second thing. XXL signs bugs because resolution is not 1024x500, but 1024x512.
(found by Agonist and me)
Line 556

["assets/prefabs/deployable/signs/sign.pictureframe.xxl.prefab"] = new ImageSize(1024, 512),      // XXL Picture Frame

1 Mebibyte = 1024 Kibibyte. And if you want 1000x this value, its not 1024x1024, but just 1024x1000. Off course.

You're welcome dude.

You're familiar with this 2 three years old problems in your plugin? :p