Put some items on the workbench(must be weapons or armor)
then get them out of your workbench to your inventory
Drop them all at the ground
Using build mode press Tab(or store) to store them back to the work shop. The game will be thinking you'll removing some items out of the settlement so the size number will go down.
Get the weapons back from the workshop
Repeat Step 2.

The size will go down as you do this, repeat as needed