For a dry dropper rig, the dropper length can be 18" to maybe 36". With an 18" dropper, you can expect your nymph to ride about a foot below the surface - great for shallow runs and pocket water. For a little deeper water, you can increase the dropper up to 36". You can go a bit longer than 36" but casting becomes more difficult.
Two notes, with a short dropper, you often end up foul hooking fish when they hit the dry, that's why I don't use a dropper less than 18".
Also, a dry dropper rig is not a deep nymphing rig since the dropper nymph has to be fairly light in weight and the dropper length is fairly short.
For a down and dirty nymphing rig with a dropper I usually space the flies 20" +/- apart. This spacing also prevents snagging the fish and allows the flies to drift naturally. I most often fish two flies - one on the dropper and one on the point, but at times fish three flies - two on droppers and one on the point.
I use weighted flies and tie patterns with different weights for my nymphing rigs. I will use split shot at times, and often stack them on the point and tie an overhand knot on the end so they don't slide off. I add or subtract shot until I get consistent contact with
the bottom.
Advantages of this rig are the flies drift in front of the fish first, the rig is infinitely adjustable without changing flies, the split shot is less likely to snag on the bottom than a point fly (if you do get snagged, you often end up pulling the shot off the line and not losing your flies) and finally the bottom moss or gunk ends up attached to the the shot rather than on the flies.
I rarely use an indy. Most times I use a sighter which consists of a few dabs of strike putty on the leader and tippet knots. You can use a section of bright colored mono just above the tippet as your sighter. A sighter system allows me to control the depth of the drift by adjusting my casts and casting without having to move an indicator up and down. I do use an indy where longer casts and mends are needed to get a good drift.
Final note, longer rods are best for nymphing. They make mending easier and allow me to hold the line and leader off the water at a greater distance. A 9' rod works, a 10' rod works better.