The gear and techniques for Euro styles varies. What a lot of people describe as "euro nymphing" is often what I have seen described as the French style. I've use that particular technique (some variation) very successfully for steelhead on small streams, and the inventors of these styles use them in international angling competition for trout, to great effect.
Start with czech nymphing. Its pretty simple and will enable you to practice fairly easily. Buy the book "Dynamic Nymphing" on amazon. It will give you everything you need to know. I ditched indicators about a year ago and never looked back. However if you find it isnt for you you want the smallest indicator possible that will still stay afloat. You have to think the more sensitive the better you will be able to detect the strike. The problem with indicators is your fly gets taken and spit back out so fast without any feedback to the indicator you never knew you had a take. But of course there are the fish that commit and take that thing under with a vengeance!
I doubt there is a standard. I think you want to go with the smallest most visible indicator that will float your fly at the depth you want it to float.
Personally I always like using the "hopper dropper" method instead of an actual indicator/bobber. Or any foam bug with a bright white or orange top so it's visible. This works for light stuff, fished in close. I would use tungsten beads on my nymphs to get them down fast. I'd also use straight tippet (preferably flouro) instead of a tapered leader, helps get down faster as well. If you go straight tippet directly from your fly line you'll have better line control as you drift your fly through the run.
Edit: foam bug for indicator because at least if a fish strikes your indicator you'll have a shot at it. You'd be amazed at how many fish will strike your indicator. Weird but it happens quite a bit.
Are you talking strictly river fishing or lake fishing? Either way, but especially in stillwaters, try to stay away from round indicators. They are harder to detect strikes. I use tear drop indicators, and I also like my indicators in the larger range to truly float heavier bugs.