Please excuse me if this is considered a daft question but I am having problems trying to sort this out - any solution or workaround would be appreciated

I am currently setting up a shop to sell school uniform -

A lot of my items that I sell are zero rated VAT up to a certain size and then I have to charge VAT at 17.5%.

Take for example Blazers

I set the product code up as zero tax class and all is well

Then set attributes / pricing for size

Sizes 25" to 38" are zero VAT however sizes 39" to 50" are Taxable at 17.5%.

I could make 2 product codes (ie Childrens Blazers 25" to 38" charged at zero and the other one Adult Blazers 39" to 50").

Ideally I would like this listed as one product and the customer is invoiced with / without VAT depending on size - hope i am not missing something obvious.

Many Thanks

Nick