Always test your party on the first pull(if you are comfortable yourself when tanking and able to take on larger pulls). If your party is not composed of high damage AoE users...definitely no large pulls. If they have good AoE, then sure medium-large, just need to test the first pull on them and see what they can do. If it takes a LONG time(i.e. BLM using Fire IV on a mob of 6+ and refuses to adjust for Fire II), then consider smaller pulls to not tax your healer too much.

If the healer is pulling...let them almost die before taking over their mobs.
If DPS is pulling...let them die as most healers won't keep them up if they are purposely pulling stuff(different if the DPS takes agro while using all the enmity reduction skills).