hide shipping options according to product size

How can I hide a shipping option when the product gets over a certain size? Eg I have items which are light and thin which i can send via 1st class mail but i also have products which are light but large which obviously i can't send via 1st class mail. however because the weight of the item is so small the option to send via 1st class mail still appears to the customer.

I'm trying to use the Royal Mail module by the way and i;ve looked all over the threads for it for a couple of days now and still can't find the answer. Please someone help me!

You would need to customize your shipping modules to check if these products that cannot be sent in this manner are in the cart and if so then disable the Shipping Method ...

Finding an identifier that is in common with all of these products would help ... or adding a new field to flag these products and then use the flag as an indicator so you know when to disable a Shipping Module or one of its quotes ...

thanks... that sounds a bit more complicated than my knowledge would allow. Can you explain how i would add a new field(?!?) to the products and how to use a flag as an indicator and how to disable a module due to a flag.

I know it must be really annoying for someone with your knowledge to have to explain in detail but any more info you can give me on this would be greatfully received. Hopefully I'll be able to figure out the rest but right now i don't even know where to start

thanks
dave

You might see if this post steers you in the right direction ...
http://www.zen-cart.com/forum/showthread.php?t=57924&highlight=add%2A+field