I think you have fully misunderstood the goal of Mappetizer. Mappetizer is a product to create [censored] web maps. With this goal there is no need to set output size but there is the need of tiling larger maps to make loading time faster (each modern webmap works with tiles). And why should i export a large coverage when i need only parts of this coverage? If you want to use the full coverage as web map then use WMS.
I do not think that Mappetizer is overpriced. Look at costs you have with Google Maps Premier or Bing Maps or other services. Mostly it begins with thousands of dollars per year. And this includes only the costs for using the API. Additionaly you have to add your developing time. With Mappetizer you do not have any additional costs.
Everbody has the possibility to download Mappetizer for testing with own data. So i think nobody buys a black box.