The stack size does not change.
I reverted test server back to vanilla stack size and then edited the config and even though it compiles and load I get this error:

  at Oxide.Plugins.StackModifier.CheckConfig () [0x0006b] in <d7255630b823455bbd8df7de83d6bf63>:0
  at Oxide.Plugins.StackModifier.OnServerInitialized () [0x00011] in <d7255630b823455bbd8df7de83d6bf63>:0
  at Oxide.Plugins.StackModifier.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000fe] in <d7255630b823455bbd8df7de83d6bf63>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <31122a27a2414cd799150f8677cf39d4>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0​

This is the config:

{
  "Revert to Vanilla Stacks on unload (Recommended true if removing plugin)": false,
  "Disable Ammo/Fuel duplication fix (Recommended false)": false,
  "Enable VendingMachine Ammo Fix (Recommended)": true,
  "Blocked Stackable Items": [
    "shortname"
  ],
  "Stack Categories": {
	      "Weapon": {
      "gun.water": {
        "DisplayName": "Water Gun",
        "Vanillia": 3,
        "Modified": 3
      },
      "spear.wooden": {
        "DisplayName": "Wooden Spear",
        "Vanillia": 1,
        "Modified": 5
      }
    }
  }
}

Did you follow the documentation/guides?

what version is this

discord? Khan#8615

Vanilla values cannot be the same as the Modified otherwise it doesn't apply the changes to those. 

Please follow the guides

I follwed the guid exactly. I was not sure about the values which is why I tried one the same as vanilla. Your example has them all the same so I was not sure if that was required

Merged post

Ok - seems like I downloaded a faulty version that was only posted here for a couple of minutes. Have the latest version and have just tested it and it works well.
Thanks for your assistance and for writing this plugin.