Validation PHP Code in Setting

[Bilderback]

This is driving me nuts.
Does anyone know how to use the Validation PHP Code box in a product setting?
I have a setting field as positive integer which I want to check if the size is in between 32 and 100.
If this returns false, I want to set a default value to be used in the product.
Would I have to initiate a plugin for this or is there an easy way to validate its size?
Example:

PHP Code:

if($size <= 32 || $size >100) { $size = 64; } 


Thanks for any help.