This mod will add lot  commands to the game.

New spawn and sub command that can change size
You can create the creature you want, such as a huge peeper.

Pickingup creatures command 

you can go find a Reaper Leviathan or whatever you want the creatures and  then catch him
put the Alien Containment.They will grow up and will breed just like any other creature in Alien Containment.

Docile creatures command
You can let Any creature not attack you because you will be friends with him.

spse [ID] [quantity] [size-X] [size-Y] [size-Z] [distance]
New spawn command that can change size

suse [ID] [size-X] [size-Y] [size-Z] [distance]
New sub command that can change size

size [size-X] [size-Y] [size-Z]
Set the size of the object you are aiming at

friend
Set you are aiming the creatures to be docile
If the creatures already has the docile it will remvoe it.

pickup
Set you are aiming the object can be picked up
If the object already has the pickup attribute, it will remove it.

playse [size]
Set the size of the player
Changing the player size can cause a lot of problems
These problems are impossible to solve, so this function is very unstable.

use

Extract the contents of the zip file into your Subnautica folder Start the game.

command example

How to use size command ? The following 4 example

spse seadragon 1 1.00

1.00 size the sea dragon

spse seamoth 1 5 

a 5 times larger seamoth

spse exouit 1 1 5 1

5 times higher Exosuit but the length and width are unchanged

suse Cyclops 0 1 1 

A 2D Cyclops looks like a piece of paper

some problems

1.giant creatures put alien containment,
When you put a Reaper Or other giant creatures in the alien containment
They will fly out because of the collision 
because their default size is bigger than the alien containment,

But don't worry, the creature will be sent back automatically after a while

2.Sea Dragon does not move in alien containment
When Sea Dragon is in alien containment, his AI won't work, so he won't move.

Github