Back when I first started playing, I would only do single pulls in dungeons I hadn't cleared before. Now that I'm more confident in my judgement, I tend to double pull first upon entering a dungeon, just to see where I stand stats-wise. If I get exploded by the mobs and we wipe? So be it, I'll just pull smaller for the rest of the run. If the healer isn't having trouble to keeping me above ~60-70% health, I go and see if I can pick up another pack. In your case I would recommend just telling everyone that you're new to tanking; statements like that let me understand the tank making small pulls.

Depending on where your tank is at in terms of level and ilevel, I would recommend going into a level 60 dungeon and telling everyone that you'll be experimenting with bigger pulls. Most 60 dungeons allow you to pull wall-to-wall without many issues in Shire gear, so you should be fine.