They come in all kinds of sizes: small enough to hang in the trailer all the way up to large enough to go over a round bale. Be sure to get a small hole hay net; the regular ones have very large holes and won't slow down your horse at all.
I used to have a Freedom Feeder mini until it disappeared from my horse's stall It's gone up in price since I first bought it, so I ended up replacing it with a SmartPak Small Hole Hay Net. This one is a lot bigger, which is good (because it holds more hay) and bad (because it hangs down too low to trailer with)
Hay nets do cut down on wastage- though my horse doesn't tend to leave a scrap of hay behind, hay net or not. You can hang the net above a plastic bin or bucket to catch anything that falls if it seems like there's still a lot going to waste.
Some horses are very hard on their hay nets and tear them apart quickly, but I think for most horses they last quite a long time.