Are you intending to be more cruisy than racy? If so, I’d leave it as aft main. It keeps the cockpit so much clearer. As to length of main, 13m springs to mind. But to be sure, push boom all the way out to the shrouds and measure from ‘windward’ side of the traveller to the end of the boom then times by 3. Add about 1m to be in your hand when running and you have your length. Incidentally I always tie a stopper knot so that the main cannot touch the shrouds if the sheet is accidentally dropped mid gybe, thus preventing shroud damage.
The genoa sheet: length not sure. Attaching – I use a simple overhand knot either side of the clew. Works for me though they are a pig to undo at the end of season should I be good enought to wash them.