If you want to change the stack sizes of items in Ultra Stacks, you can set custom stack sizes by modifying your Game.ini. Here is a sample of how to change the stack size of wood in Ultra Stacks:

ConfigOverrideItemMaxQuantity=(ItemClassString="PrimalItemResource_Wood_US_C",Quantity=(MaxItemQuantity=20000, bIgnoreMultiplier=true))

With this setting, this would increase the stack size of wood to 20000.

You can also add the following setting to change stack sizes overall by adding this to your GameUserSettings.ini:

ItemStackSizeMultiplier=2.0

With the setting above, all stacks would by multiplied by 2 (e.g. resources that stacked to 5000 would now stack to 10000).

To reduce stacks, you can use the following:

ItemStackSizeMultiplier=0.5

This would reduce all of the stacks in the game by half.