The first chapter of Ezekiel describes a vision which Ezekiel received from the Lord. Your question references verse 26: “And above the firmament that was over their heads was the likeness of a throne, as the appearance of a sapphire stone: and upon the likeness of the throne was the likeness as the appearance of a man above upon it.”
We know from Exodus 33:20 that no man can see God and live, so we can reason that Ezekiel did not actually see God, but he was given a vision showing a representation of God in a way which Ezekiel could understand.
You are correct in saying that God is not a man. In fact, man was made in God's image (or likeness) as stated in Genesis 1:26: “And God said, Let us make man in our image, after our likeness.” This is not referring to God’s physical image, but God’s characteristics of love, justice and wisdom. God’s ways are so much higher than man’s that in order for God to communicate with us, He needs to condescend to man’s level and put His word and His visions in ways man can understand. If God had shown His true appearance in this vision, it is likely that Ezekiel would not be able to describe it nor identify the being as God.