I think you are close to correct.
Try adding a parameter in simple view for the recordset first... then go to advanced view so you can see the syntax and add a more complex sql statement.
Sounds like you would need to filter by two ID's to get the correct size, weight, and price, but it should be possible.
That is correct. eCart "fills the gaps" to allow you to create eCommerce sites. It doesn't reinvent the wheel for what already exists and can work with custom code to add any and all possible features. A strong knowledge of php, Dreamweaver, and mySQL is certainly important if you are trying to stretch the capabilities of the tools.