How tall should seedlings be to be considered healthy and good?

The height of the plant itself isn't an indicator of how well a plant is doing. Different strains and even different plants of the same strain will have different spacing between the nodes where branches form. Some have bigger spacing resulting in a taller plant and some have shorter spacing making them shorter overall. To judge how healthy a plant this small is in comparison to another plant you are growing to count the nodes and observe overall growth. Are the leaves a good size? If its a bit later in the grow when the plants are bigger, do the leaves have more than 3 fingers? Sometimes the number of fingers is purely genetics but if you know that plant has been putting out 5-10 fingered leaves and then all of a sudden it starts throwing 1-3 fingered leaves it can be a sign of stress. So with that in mind, both plants look to be at a similar level of growth and size besides the minor height difference. They both seem to have 2 nodes of true leaves formed and the smaller one looks to be working on its 3rd node while the other one its kinda hard to tell if it is. So all in all, they are both doing well and there's nothing to worry about so far.