G'day Pete,
Good question and this has more than one answer. To scale down would work exactly the same if you scaled the catching section and the water flow down uniformly.
If you scaled down the length to half, and had the crate go to the end of the box, with the Mesh boiler section cut in half, you could still run the same water pressure and catch all the gold. This would however require a lot more clean outs during the day, as the number of traps is also cut in half.
The biggest problem in halving the length of the box, is maintaining water speed in a single length of sluice. My recommendation if you want to halve the 1500 to a 750 sluice, would be to make a 2 tiered sluice, as the top box gives you the required water speed to feed the shorter lower sluice more efficiently.
To run a single sluice banker efficiently I wouldn't recommend shorter than 1200......hope this helps.
Cheers Wal.