If the self-made plastic non-functional decoration does not use 3D printing, is it also limited by the size of the plastic sheet in this rule?
All rules governing non-functional decorations can be found within R12, partially quoted below, with a portion bolded for emphasis. Plastic used as decorations is not subject to R9, as long as inspectors determine that it is non-functional, as described in R12.
<R12> Decorations are allowed. Teams may add non-functional decorations, provided that they do not affect Robot performance in any significant way or affect the outcome of the Match. These decorations must be in the spirit of the competition. Inspectors will have final say in what is considered “non-functional.” Unless otherwise specified below, non-functional decorations are governed by all standard Robot rules.
To be considered “non-functional,” any guards, decals, or other decorations must be backed by legal materials that provide the same functionality. For example, if a Robot has a giant decal that prevents Discs from falling out of the Robot, the decal must be backed by VEX material that would also prevent the Discs from falling out. A simple way to check this is to determine if removing the decoration would impact the performance of the Robot in any way.