With the supplied particle system, the size is uniform, therefore we only store a single value per-particle (it would be a waste of memory to store the y and z values).
Therefore, your script should only set the sizes.x property.

To set a different value for each axis of the particle system (non-uniform) enable the 3D Start Size checkbox in the main particle system settings.

public void Execute(ParticleSystemJobData jobData)
{
var sizes = jobData.sizes.x;

for (int i = 0; i < jobData.count; i++)
{
sizes[i] = 1.0f;
}
}