Y position for player to be considered inside radius of Lingering Potions?

From Minecraft Wiki:

The cloud starts with a radius of 3 blocks, decreasing to 0 over the course of 30 seconds. During the time the cloud is there, any player or mob that walks into it after the first second will get the corresponding status effect; this decreases the radius by 0.5 block immediately, causing the cloud to disappear more quickly.

At most how high should a player be (the player's Y position) from the cloud of Lingering Potion in order to be considered inside the radius, thus getting the effect?

All that matters for the player's Y position is whether or not their hitbox intersects with the AreaEffectCloud entity's hitbox. This does mean that your head being within the cloud also counts, rather than just your feet.

The cloud's hitbox height will always be 0.5 blocks, regardless of radius. For comparison, the player's hitbox will be 1.8 blocks tall under normal circumstances, 1.65 when sneaking, 0.2 while dead or sleeping, or 0.6 while gliding with elytra.

You can view hitboxes by pressing F3 + b. Image example, where the player is intersecting with the cloud: