How big is the area of effect for the Ice Knife spell's explosion, when the spell targets a Large creature?

Question

The description of the ice knife spell states (emphasis mine):

You create a shard of ice and fling it at one creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 piercing damage. Hit or miss, the shard then explodes. The target and each creature within 5 feet of it must succeed on a Dexterity saving throw or take 2d6 cold damage.

When hitting a Medium creature, the area within 5 feet of it covers 8 squares on a 5-foot grid. If you hit a Large creature, it covers 16 squares for the same spell. RAW am I reading the spell right, can you damage (potentially) twice as many creatures?

Answer 1

Not exactly

It also affects creatures within 5 feet above and below. So that’s 27 5×5×5 spaces for a Medium/Small creature (including the creature’s own space - it might have/be a rider), or 64 spaces for a Large creature, and so on.

Answer 2

For practical purposes, yes

Ice knife received an erratum and no longer uses language about a radius,¹ so it cares only about the distance from target creature to adjacent creatures. That means that any creature within five feet of the target creature will be affected, just as the spell says. With a larger creature, you will affect a larger area.

How that translates in play depends on whether you play on a grid or not. If you do, then even though the creature does not fill the entire grid cell, any adjacent square will be in range, as the rules advise you to count that as 5 feet (PHB, p. 192 – "Variant: Playing on a Grid" sidebar):

To determine the range on a grid between two things — whether creatures or objects — start counting squares from a square adjacent to one of them and stop counting in the space of the other one.

As Dale points out, in the unusual case that you deal with 3D combat, you will affect a larger volume, and also hit the cubes above and below the target creature.²

If you don't use a grid but still use miniatures, then you measure the distance between the creatures. If you use narrative combat, the DM will determine which creatures are within five feet of the target.

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¹ There is some discussion of whether effects that spread in a radius around a target creature (typically the caster, such as spirit guardians or the effects of a scroll of protection) originate from the overall creature or from a point you need to pick. This is not unequivocal, so it will require a DM call (there is a strong majority vote on this site that it will originate from the overall area the creature occupies). Because ice knife does not use the radius language, it avoids these issues.

² If the target was Huge, then you would affect an area of 25 tiles or 125 cubes, and if it was Gargantuan, you would affect 36 tiles or 216 cubes.