Hi natterjack - I don't really think there is such a thing as a standard portion size and as you will see from previous posts, tray sizes are different and I guess the one you measured represents the size of that establishment and tray manufacturer.
I've recently moved from foil trays to clear plastic trays, costing just over a quid for ten trays with lids. I like them better than the foil ones cause they can be used in the microwave. I've just filled mine with water and it is measuring 600 ml/
Having said that, I reckon 500ml is a good ball park for a portion size. I just happen to be a greedy bugger!