i use notepad + to edit the " loot.xml " in my Config file, found at :\Program Files (x86)\Steam\steamapps\common\7 Days To Die\Data\Config your install may be different. the containers all have the same basic definition, like this for player storage
as you see, i've edited mine to 12 columns by 10 rows. it will only change newly placed containers in a save, will not change existing containers. and if i forgot something, someone will come along and correct me