Quantity of Orzo when using other ingredients
In my experience orzo behave much like rice, in the sense that grow a lot in volume after cooking.

I'm supposing that the orzo salad will be an apetizer or at least not a main dish, and you are talking about pearl barley which requires only 20-25 minutes of cooking with no pre-soak(but remember to wash it before cooking), so you can count about 50g per person.

50g x 25 = 1.250Kg (which can be rounded up to 1.5Kg just to be safe in case someone want a second dish)

If I recall correctly for every cup of orzo you add at least a cup and a half of water, so you'll need at least ~3.50L of water for the cooking