As your question is posted in the Wireless forum, it's not fully clear if you refer to wired or wireless clients in your question.
If you connect a controller (that does broadcast filtering/conversion) with many wireless clients, you should, in general, take the subnet as large as possible. Check the Single VLAN Architecture for WLAN VRD for more information and backgrounds.
In a pure-wired environment, I agree keeping the broadcast domains small (just a hundred or few hundred devices per VLAN) is considered best-practice for the reasons mentioned in the previous answer.