Stacksize

The above items are the ones automatically included in the config file to use the new stacksize value, but this can be changed by editing the "stacksize.cfg" file along with the new stacksize value you wish to use. Other mod items can be added to this as well, but require the mod's registered item name.

* Enchanted books don't always stack nicely. Of course only certain enchantmented books can stack, like sharpness 1 can stack with other sharpness 1s. But sometimes you have to put them into a chest, close the chest, and re-open the chest to stack those two books. NOTE: using 2 or more enchanted books on an anvil will not produce a higher effect, but only consume all the enchanted books.

Adding other mod items to the stacksize requires the exact mod's item name. For example "modname:item_name". If you don't know what the mod item name is, you can find the mod item name by hitting F3+h. Then it'll show extra information in the tool tips, including the mod item name and ID. Open your inventory. Locate the mod item you wish to make stackable. Mouse over that item. You will see the default item name followed by a #NUMBER. The NUMBER is the item ID number. Below both of those, you will see grey text. The grey text is the mod item name. Copy that mod item name including the mod name into the Stacksize.cfg to change that item's stacksize. You can place it in the S:"Item List" as a new line below the other default minecraft items.  [Thanks to DRTR0]

Another more complicated way, is to look at the fml-client-latest.log and search for the text "Registry add: " after starting minecraft with that mod once.