i was at 150 space for knight and 100 for equipment, but for convenience i increased it recently to 220 for knight and 150 for equipment, i found it was taking up alot of time micro managing it. As found a beginning it was easy to have less capacity, but as more knights and event equpiments is collected it is not worth the micro managing.
Being a collector, I currently have 121 knights (including all 2* and 3*), and because i save up the lvl 100 Manyus and the bloom dragons, i was constantly upgrading knight to do campaigns. ie you can not campaign with your space overloaded.

As to gift box, i do not worry if i am over 100 spaces, never had issue with it, just collect stuff until it is under 100 mail items.