I'm not surprised that you're having trouble getting responses to this. It does push the boundaries of what Zen Cart can currently do somewhat. It can be done but it require a fair bit of custom programming.
There are two main challenges. The first (and easiest) is the layout, the other is the underlying need for an attribute to which you can attach quantities, and then go a step further and support discounts based on those quantities.
That you wish to not to apply to all products actually males it a smidgen easier as it says immediately that something slightly exceptional is going to be needed for The first is most easily achieved by creating a new product type. This will cause the template system to look for a special product_info page related to that specific product type, which is where you would bypass the standard attributes layout to achieve the table-based version appropriate to this product's attributes.
More complex is addition of a quantity-supporting attribute. This is not part of core Zen Cart and goes into some tricky code, but it can be done. There's an example of it here, which although looking superficially very different (not least because the quantity boxes are hidden - click on any charm twice and you'll see it added to the product summary twice), is fundamentally what you're looking for. To this would need to be added pricing that takes account of the quantity entered for each attribute, which would be less complex.
That would get you the precise solution for which you're looking. However, there's an alternative approach which makes more use of what Zen Cart already has and overall would achieve approximately the same result, but with less effort and cost.
This would be to create each different-size as a separate product, pop them all onto a category that represents the product and create a custom product listing page for that category that lists them as you wish and takes advantage of Zen Cart's built-in "multiple add to cart" and "quantity discounts" features.
In my view this would have some advantages over the more complex single product option. Each size would show as separate item in the cart and on invoices (which is closer to what I would expect as a customer) and it would be possible to tweak quantities in the shopping cart, which would not be possible with the single-product approach without yet more custom development.
This is a very high level explanation but I hope helpful is showing what options are available to achieve your requirement.
