Crested gecko enclosure size

The chart

Note the units: weight, not age. Growth rates vary and weight is the thing you can actually measure, which is why the published guidance uses it.

Why height rather than floor

The published guidance states that crested geckos are arboreal, meaning they are a tree-dwelling species, and generally prefer terrariums that are taller than they are wide. That is the whole derivation, and it is worth contrasting with the leopard gecko, where floor space is named as the most important consideration precisely because that species is terrestrial.

Same family of animal, opposite constraint. It is a useful illustration of why a generic "gecko enclosure" recommendation is close to meaningless.

Why the hatchling grow-out matters

Two reasons, both practical. A hatchling in a large planted enclosure is genuinely hard to find and therefore hard to monitor - and weight tracking is how you catch a problem early at that life stage. And there is a lighting hazard: putting a UVB lamp on top of a tiny grow-out cage will likely expose the animal to dangerously high UV, because the distance does not exist. The published alternative is a forest-strength compact bulb in a 5.5 inch dome on a lamp stand, off at night.

Going larger

More height and more volume are both genuinely useful for this species, and a larger planted enclosure is a better environment. The constraint is the temperature ceiling: a bigger insulated box is harder to keep under the 85 F sustained limit, so measure rather than assume. The temperature page covers it.