Just to say, whether you are aware or not, raid size is independent of difficulty.
In any raid, you will always want 2 tanks, ~20% of your raid should be healers and the rest DPS.
10 man should be, 2T, 2H, 6D. 20 man should be 2T, 4H, 14D.
NB: Depending on player skill you may be able to run more/less healers.
MMOChamp also has a Raid Comp link
here (clicky) that enables you to make sure you have the most amount of buffs possible (having all buffs isn't essential by any means, but if you can you should. You almost definitely should have at least one class that can Battle Res (Druid, DK or Warlock) and one that can pop Heroism (Shaman, Mage or Hunter*).
Now down to raid leading. You yourself need to be fluent with tactics, you need to know how you want to execute each fight and know the mechanics of each encounter. You also need to be able to multi-task pretty well, you're going to need to watch the raid and timers, whilst executing your own rotation.
You will also need to be calm and collect; wipes happen and you need to ensure you personally keep the situation under control. The raid environment is usually reflective of the raid leader so screaming blue murder at anyone that wipes you usually leads to an even worse run, whilst doing nothing will obviously achieve nothing.
Be tough-skinned, be open and don't be scared to be assertive. Call out when the raid needs to move in good time, call out DBM timers if the group is fairly new, constantly interact with the raid (socially and about the fight).
Raid leading is a tough responsibility, but it is thoroughly rewarding if done correctly.
Good luck!
Source: Raid Leader of my own guild.