I’m using the Universal Viewer plugin [ https://omeka.org/add-ons/plugins/universal-viewer/ ] and uploading a number of high resolution photographs.
Could anyone could advise me on optimum dimensions and file sizes to use with this plugin?
There is no limit on the image size, only the one of the server. Nevertheless, for big images, you should tile them with OpenLayerZoom (http://omeka.org/add-ons/plugins/openlayerszoom/).