Same here. Although the M92 ist quite expensive, it is still rather cheap if you consider it a tool, not a toy. For the same functionality and a 13" screen, I would gladly pay up to 300€ more only for the bigger screen.
But keep in mind that the quantity of M92s produced is small once you compare it to the other brands, especially the Kindle. The M92 serves mainly customers who use it as a tool. If they "split" the market further by offering a 13" screen model, the quantity of produced M92 and the bigger device will be lower then the quantity of M92s prduced nowadays. In the end, that might prove risky for the producer since the lower the quantity produced, the higher the cost per unit produced.