We don't currently allow you to change the tile size. But, I'm not sure that you want to. Increasing the tile size will affect the performance of users viewing the data. They'll need to download larger tiles to satisfy pan&zoom changes.
HDView, Google Maps, Bing Maps all use 256x256 tiles for this reason. I think that the best solution for this is to put all of the tiles in some sort of container, for example a zip file. Then build a server-side component that extracts the
appropriate tile from the zip. We don't have such a thing available though, but it seems like something that would be pretty simple to build.