Our products are categorized by Gender (Mens, womens, youth, etc).
We create the products using Configurable Option which create individual variance by size. For Ex. TShirt-Small, TShirt-Large, etc
For this project, we want to customize the tax calculation based on Gender and size.
if the Product category is Youth and the size range is from small to large - tax exempt on Provincial tax. (we've done this part)
if the Product category is Youth and the size range is from xlarge and above - should apply normal tax calculation (GST + PST or HST depending on the province).
If you are not familiar with Canadian taxation we can guide you. I prefer that this be done through magento tax / product configuration rather than manipulating the code but if not possible then we would of course willing to work on the customization.