What kind of plant are you going to add to the terrarium? Moss and epiphytic plants don’t need much soil, while “normal”(terrestrial) plants need different soil thickness depending on how deep the roots grow.
In addition, with a terrarium of such size, I would recommend having a thicker drainage layer. Larger terrariums need more water to create the high humidity; a thick drainage layer could store more water without soaking the soil.
What kind of plant are you going to add to the terrarium? Moss and epiphytic plants don’t need much soil, while “normal”(terrestrial) plants need different soil thickness depending on how deep the roots grow.
In addition, with a terrarium of such size, I would recommend having a thicker drainage layer. Larger terrariums need more water to create the high humidity; a thick drainage layer could store more water without soaking the soil.