The carrying capacity can be viewed as valuable in the sense that it enables you to send less units to complete the task, thus the risk in unit number is reduced if someone attacks you. Other things you may want to consider is how valuable that unit type is to you specifically. 

For example, light infantry tends to provide the best resource cost to defense power ratio, but at the same time if your army is large it starts becoming challenging to feed too many light inf units, because they consume a lot of grain relative to their power. 

Additionally, it might be best to send in a hybrid of units for defense purposes, to counter someone scouting and then sending attack units which your defense doesn't perform very well against. 

And of course, you will probably want to send units which you developed agreements and unit xp for, to make stronger (same as PvP concepts).

I'm sure I missed some things, but hope this helps. 

Emporium

Usually the best thing is to upgrade fully and pile up the things you need before engaging at Emporium.

Apart from ranking up army, signing agreements, activate dominion and boost offence or defense for a successful hold and collection of your resources you will need to give attention to few upgrades to get the full benefit and advantage from your stay at Emporium.


In Academy 

In General Abilities

In Dominion at level 15

In Market are available Boosts to purchase.

To the Emporium is wise to engage when the tournaments are on to gather points and take full advantage with a good pay out.

Depending on your own strategy you can use the Emporia to either gather resources or just destroy enemy units.