In Age of Sigmar you have a great mix of Warscrolls which can be almost any size you desire, in Matched Play this is somewhat limited but large thirty to sixty man units are still available for some factions. On the other end of the scale most units are three to ten models for a minimum. In this post I'm going to list the pros and cons of small and large model count units, for the sake of this exercise imagine a unit of thirty models vs three units of ten models.
Multiple Small Units
• If you split a unit up you now have a leader per unit which in our example is three compared to one. As leaders of units always have a small buff this can amount to a noticeable effect over a whole army.
• In the same light as leaders, small units also have access to more Banners or Icons, which in general won't make much difference to being in one large unit except in the case where they have a unique ability such as the Hellstriders of Slaanesh Enrapturing Banner.
• When charging multiple small units you have more chances to succeed with at least one or more of your units when compared to one. This is an affect of dice variance. Essentially you have a more reliable chance of getting models in to combat however it may not be all that you want, compare to charging a single large unit which either makes the charge or does not.
• Multiple small units really limit the maximum amount of models that can be killed by any one unit. A war machine shooting with 2d6 mortal wounds is much more effective against a unit of ten or more compared to a unit of five for example.
• Depending on the scenario objective you may need a larger amount of capturing units. If you only have one large unit you can only capture one objective where three smaller ones can obviously capture three, if available.
• You can be more disposable with multiple units as they aren't worth as much. Using a small unit of five as roadblocks is much better than losing a larger unit.
Single Large Units.
• Units gain a bonus to their Bravery for every ten models, which can be quite important for the survivability of weaker units. Remember though, this bonus is only in the Battleshock Phase so won't protect you against abilities used in other phases(Thank Bravery One for that one).
• Most of the weaker units have a bonus the more multiples of ten they have, often a to hit, to wound or attack bonus which can make these seemingly weak units very dangerous, especially when combined with other buffs.
• An obvious benefit or the larger unit is their resilience in numbers, more wounds need to be taken before that signs unit is wiped out which can be useful for holding enemy units in place.
• The area of control of larger units can be useful as you can stretch out in a thin line formation to ensure contact with the enemy and then pile in around them. Multiple smaller units could easily be destroyed and moved through in one round of combat.
In summary there are uses for many different unit sizes and likely many more than what I've suggested, you just have to figure out the role your unit is going to play in the battle and adjust its size to optimise.