It's under settings...
settings {
inventory_items_limit_var = 10
inventory_weight_limit_var = 10
inventory_worn_items_limit_var = 10
}
I added a full code snippet to the cookbook.
It may be confusing because the game has a settings section, but each theme has a settings section too. I think your problem is that you are looking in the theme settings, not the game settings. Sorry about that.
I've updated the document, which should prevent this sort of confusion for the future: