possible to do validation in not current model? where should I put validation??

For example

There are 3 tables "categories", "products" and "sizes", and I'm
working on product page (product.php, products_controller.php,
product.ctp).

The database looks like this.
sizes:
-----------------------
| id |
-----------------------
| category_id |
| size_name |
----------------------

products:
-----------------------
| id |
-----------------------
| sku |
| category_id |
| product_name |
| size_id |
------------------------

categories:
---------------------------
| id |
---------------------------
| category_name |
---------------------------

Product page can add/edit product:
Before I save new product the page also can add new size.

I can verify a product data such as sku value from the model but I
want to also make sure,
user can add new size name if it's not exist in the system(not exist
in size table).

I want to put validation code in model.

Should I put validation code in size.php or product.php

Also if I want validate this then I have to pass category also.
(same size name can be exist if it has different category id)
How can I pass specific value to model so I can set vilification in
mode and not controller?

Or in this case I should just put code in controller?

thanks
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