Yes. Stitching the tiles together is relatively easy. Depending on the specific tiling scheme, you may be able to do so without any depth of knowledge in geography.
That said mapnik [1], the rendering engine used to generate tiles, is a general purpose geographic data renderer. You could use tilemill as a tool to generate the map style then use mapnik directly to render your map at any pixel or geographic dimensions desired.
That said mapnik [1], the rendering engine used to generate tiles, is a general purpose geographic data renderer. You could use tilemill as a tool to generate the map style then use mapnik directly to render your map at any pixel or geographic dimensions desired.
[1] http://mapnik.org/