Upsizer Mod (1.18.2, 1.16.5) – You Can Now Stack Up to 64

When deciding what you should up-size keep the following three categories of items in mind:

• Upsize-ready (Harmless) Items: These are items that already support multiples per slot but have some imposed limit in the interest of “fair” or “balanced” game-play or some such thing. Examples include: signs, banners, eggs, ender pearls, buckets. Up-sizing these should not trigger any issues unless there is another mod installed that requires the vanilla stack size (which would be odd but you never know).
• Damageable Items: These are items that can take damage and are probably limited to one-per-slot as each damage level is effectively a new type of item. Up-sizing these sort of items is most effective for storing multiple new or undamaged items. You should still only use these items with one per inventory slot. In fact, when crafting new items don’t even have a damaged version in your inventory as Minecraft might merge the new items into the same slot as the damaged one(s). Examples include: tools, weapons, and armor.
• Contained/er and other special use Items: These items may be limited because of how they are used. Up-sizing these items is also just a way to increase your storage of unused items as Minecraft is often hard-wired to assume use of a single unstacked item at a time. Examples include: cakes, stews, filled buckets, filled potion bottles, and enchanted books.

When you up-size items that are usually limited to a single item per inventory slot, pay attention before using the stacked items as a single item (this goes for using the item directly or indirectly via crafting or smelting). Or better yet, save yourself any pain and just don’t do that. Store the items stacked, always unstack to a single item before using.

Potion Bottles

Do not up-size filled potion bottles (aka water bottles) unless you don’t plan to use them for brewing. Minecraft requires that the potion items have a max stack size of 1; anything else is rejected and you cannot put that item into the brewing stand’s inventory slots.

Item Filters and Sorters With Hoppers

If you have built standard item sorters or filters with hoppers for some of the items you want to up-size, you might need to adjust your hopper filters. In particular, you will need to make sure the number of filler items you use in the filter hopper is adjusted for the new max stack size. Normally, items like eggs and ender-pearls stack to 16 and so you’d use perhaps 12 filler items in your hopper filter; if you increase the max stack size for eggs or ender-pearls, you will need to adjust the number of filler items up (the exact change, if any, would depend on you filter’s design and the max stack size you’ve chosen).