Indeed, product dimensions could be crucial for carrier-calculated shipping rates. As far as we know, it is especially important when using UPS-provided shipping methods. Unfortunately, it is not yet possible to specify the product dimensions in Ecwid, sorry. But we understand the troubles Ecwid users may have due to the absence of product dimensions controls, so we will certainly add them in one of the future Ecwid versions. There is a related suggestion in our ideas base:
Please vote for it and we will notify you when it's ready.
Meanwhile, there are two workarounds for this situation: using an additional per-product fixed shipping charges or setting a bigger weight for the oversized products. Please refer to this post for the details:
I understand that the both workarounds have some drawbacks though. In your case, as far as I can see, the fixed per-product rates don't meet your requirements because they apply regardless of the selected shipping method so that even a free local pickup shipping could become non-free for some items in your store. Well, that is why they are called 'fixed' - they are often used when a special charge is required for some item in the cart, for example, if the shipping cost should include a special packing price for the product.
What do you think about the second solution (increasing the weight of some products to compensate for their shipping charge)? Does it suit you?