Why did he request Chachmah and Mada?
Malbim (7): Chachmah is knowing how to conduct in matters that involve good and evil. Mada is clear knowledge of everything that is known through the senses or is self-evident. To do justice, one must know the laws ? what is just and what is unjust, what is good and what is evil. One can know this only from a Kabalah from Hashem. Also, one must be able to discern the Emes and Sheker in people who come with a dispute, like the two women arguing about the baby. Shlomo perceived the truth via Mada of sense and Muskal Rishon. Chachmah and Mada are needed also to go and come in front of the nation in leading them.
Why does the size of the nation obligate that Shlomo receive Chachmah?
'Rashi': They have many matters that come for judgment, and I do not have time to investigate each. Who can judge 1,000 cases in a day, if not a Chacham with Ru'ach Elokim?!
Malbim (Melachim I, 3:8): My small Chachmah is not enough to lead a nation so great, both in quality ("Amcha Asher Bacharta") and quantity ("Am Rav Asher Lo Yimaneh").
