66% is the size I settled on. Look good. Relatively easy to make. The hemiolia is possible - stands in as a pentekonter/scout.66% is nearly 50 and gives nearly four times the playing surface.
An unexpected bonus is that the models stand alone and no base is needed. The oar-banks just need bending down a bit. I painted the undersides with pva glue and sprayed the models with matt varnish. The finished thing is quite strong and does not need such care in storage as do full-size models on bases.
Deck crew at this scale could be a problem but the 6mm card figures I made for full size models actually look quite ok. (phew)