Is divorcing one's wife considered an injustice to her if it is due to physical characteristics that were only discovered after marriage because they had not met, knowing that she chose divorce after being given the choice between it and polygyny, and what mahr must he pay?

If the wife is righteous and upright, divorce is not recommended because short stature is not a defect that necessitates it. If you decide to divorce her, there is no sin upon you, as divorce is permissible, especially if there is a need for it. The divorced woman is entitled, after consummation, to the specified mahr or the mahr of her peers. It should be noted that it is not sufficient to merely see a picture of the prospective fiancée; rather, it is preferable to look at her directly, as the Prophet (peace and blessings be upon him) said: "Look at her, for it is more likely to engender affection between you."