Why are my plants so small?

[NotAFridge]

Does the size of my grow room affect the size of my plants?
I have been growing autoflowers in a converted double door fridge, but they never seem to grow very big (around 1 foot tall)
I do use LST training quite heavily as I do not have a lot of vertical room, but I expected my plants to basically fill the entire space.
Room specs and equipment used are:
82cm W x 54cm D x 92cm H (2.6ft W x 1.7ft D x 3ft H)
Light: Dakason KS-1000
Extractor: Prima-Klima pk100-tc
6in Oscillating fan
4in x 12in carbon filter
4in Ducting
Grown in 20L (5 Gallon) fabric pots
Medium: Canna Terra professional
Nutrients: Full range from Canna

 

[quirk]

Your light pulls 100W. You did well.

 

[PLreef]

At 100 watts I think you did alright. Up the wattage and see what happens. Light = food

 

[gbauto]

Auto's can be quirky.
I've had some that were only 8-10" tall, and others that were 4'.
Overall, your setup should work.
The fixture you're using is a bit under-powered for that space.
100 watts of 3 generation old diodes won't produce enough photons to flower that space effectively.