He may have asked about your height because he wanted to get a boost to his ego.
He may already know that he is short.
By standing next to you to compare heights, he may have been trying to reassure himself that he isn’t as short as he believes himself to be.
It is unlikely that he is sizing you up.
On the contrary, he was most likely sizing himself up.
This can also be said about how he has also asked you about your weight.
When you told him what you weighed, he said that he could pick you up.
Again, this may be a guy who may already know that he is diminutive or small in stature and often wants to feel reassured that he isn’t as small as he thinks.
Given his smaller size, both in height and weight, he may have friends who often tease him about it as well.
That does affect his ego.
When he is able to compare himself to you and feel bigger and stronger as a result, this helps to boost his ego and make him feel better about himself.
With this in mind, it is unlikely that he is flirting.
He just wants to feel better about himself.
If he stares at you from a distance with no expression and his lips apart, he may be curious about you to a certain extent.
You may remind him of someone or he may be wondering about what kind of person you are.
This may or may not indicate romantic interest.
You would have to start engaging with him in real get-to-know one another conversations in order for you to determine whether there is true romantic interest on his part or just mere curiosity.