You can setup your allocation rule based on the package size. 

1. Create allocation rule under Setup > Stock > Allocation and issue rules

2. Create a new allocation rule where "Sort by Coefficient" is set to be "Descending". What this will do is, it will allocate you inventory based on the Package size where the largest would get allocated first. 

3. Once you created the allocation rule, go to the Common Data-> Products-> product category, and find the category of the item that you are picking. 

4. Under the issue tab, depending what you are picking for, if for sales order, apply your new allocation rule under the "Order". If you are picking for work order, apply your new allocation under "Work Order"

Please note that this change will apply to all products under this category. There are a lot of different setups you can try with allocation rules. What I demonstrated is just an example. let me know if it helps!

Thank you! 

In addition, on the Allocation rule, "PAC complete" should be ticked also.

You shouldn't tick 'PAC complete' else there will be a shortage of 5 instead of a partial allocation as requested.