Well there is a cheesy way to reduce all ships and stations size 4 steps
Step 1, creative mode infinite resources and stuff to have no limits
Step 2, Build what you want using the size reduction option at the main build from blueprint option.
(usually 2 size difference up or down, in this case down is what you want.)
Step 3, go to the modify all ship option and scale it down by 50% each time until you get the size you want.
(warning this can cause issues with power and such as some things dont scale well either up or down.you may need to modify it to fix issues.)

Step 4. SAVE Blueprint at that size you wanted. You now have a size you want.

15 slots FML im in that mid game grind how do i reduce that number to like 8 bro.