Typically you control the flow of water into the spa using a diverter valve at the equipment pad where return water is split between the spa and the pool. The larger the spillway opening, the more water needs to be diverted to the spa to provide the volume of flow to make the spillway look like a sheer waterfall and not a dribble down the tile face. If the opening is too big, then you will have to divert most of the water to the spa and much less will go to the pool returns; in the extreme you'd have to divert 100% of the return water to the spa. The problem with that is the spa is much smaller volume of water than the pool and so you'll be turning over the spa volume many, many more times than the pool volume. That is bad for pool filtration and can lead to stagnant dead spots in the pool water.
I typically have to split my diverter valve 50/50 with the spa/pool returns to get my 18" spillway to run right. I could run the pump at a higher speed but that costs more. So if I had a 24" spillway, then I'd have to really starve my pool of water in order to get the spillway to run. I'm actually planning to put an actuator on my spillway control valve so that I can turn it on and off from my remote. The spillway runs way more than it should and the excess aeration does affect pool chemistry. So, if you have the ability to add it to your design, you might want to automate the spillway.
18" works for me. I have a 3HP IntelliFlo VS pump. So depending on your plumbing setup 18" would probably be fine. Plus, 18" is three 6" tile widths. If you go to 16" you're going to have to snap a tile or use smaller tiles and it may look weird.
Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006