It sounds like a really interesting project. I have a mural project on-going as well and hope we can exchange information. If you haven't already, I'd recommend you update to the latest version of Photoscan (currently v. 1.2.1). It seems to produce orthomosaics with fewer artifacts and better resolution, based on the results I've seen so far.
It's not necessary to generate texture for the mesh in order to generate an orthophoto. However, the resolution of the orthomosaic is partly dependent on the quality of the mesh. The size of the pixels in the "Export Orthomosaic" dialogue box is set by default to the highest possible resolution, so I wouldn't change these values. If the image is too large, it might help to break the orthomosaic into blocks, which you can choose in the Export Orthophoto dialogue box.
First, I'd generate the highest quality point cloud that your computer's memory is capable of using to generate a mesh (it's the step most demanding of RAM). It's better to start with the most points possible in the dense point cloud and then decimate the mesh if necessary, rather than to start with a less dense point cloud and generate the mesh at the highest possible settings. After you've generated the mesh using the highest quality settings your computer's memory will allow, I usually use Tools->Mesh->View Mesh Statistics->Fix Topology to repair defects in the mesh. If there are holes in the mesh or occlusions, you can also use Tools->Mesh->Close Holes. Then you can export the orthomosaic using the best quality settings. I've found the tutorial on generating orthophotos is a useful reference, also.
Best of luck,