"When God says to the Messiah, "Sit thou and my right hand...." What does He mean? Considering God is a Spirit, and cannot be seen, nor does He have a human body, how could Christ sit at His right hand, if he doesn't literally have a hand?"
I will try to answer your above question as best as I can.
If I say to you, "You are my right hand man". I could mean literally "you are the man sitting/standing on the right side of my body" or most correctly in a symbolic way, "you are my most trusted man that I can rely on". But whether or not I mean literally or symbolically, I do mean I have a right side of my body. Taking God to be an unseen Spirit, he could easily form a human body just like he created Christ in a human body and would be able to distinguish his right hand from his left hand.
I have compiled a list from an on-line bible on God's right hand. The list is many and I have shortened it. Some do suggest interestingly a literal God's right hand to me (see those highlighted in red, the bold words suggest literal meanings):
⁃ Exodus 15:12 You stretched out your right hand and the earth swallowed them.
⁃ Psalm 44:3 It was not by their sword that they won the land, nor did their arm bring them victory; it was your right hand, your arm, and the light of your face, for you loved them.
⁃ Psalm 74:11 Why do you hold back your hand, your right hand? Take it from the folds of your garment and destroy them!
⁃ Psalm 89:13 Your arm is endued with power; your hand is strong, your right hand exalted.
⁃ Psalm 98:1 [ A psalm. ] Sing to the LORD a new song, for he has done marvelous things; his right hand and his holy arm have worked salvation for him.
⁃ Psalm 110:1 [ Of David. A psalm. ] The LORD says to my Lord: "Sit at my right hand until I make your enemies a footstool for your feet."
⁃ Psalm 137:5 If I forget you, O Jerusalem, may my right hand forget its skill .
⁃ Isaiah 41:10 So do not fear, for I am with you; do not be dismayed, for I am your God. I will strengthen you and help you; I will uphold you with my righteous right hand.
⁃ Isaiah 48:13 My own hand laid the foundations of the earth, and my right hand spread out the heavens; when I summon them, they all stand up together.
⁃ Isaiah 62:8 The LORD has sworn by his right hand and by his mighty arm: "Never again will I give your grain as food for your enemies, and never again will foreigners drink the new wine for which you have toiled;
⁃ Jeremiah 22:24 "As surely as I live," declares the LORD, "even if you, Jehoiachin son of Jehoiakim king of Judah, were a signet ring on my right hand, I would still pull you off.
⁃ Matthew 22:44 " 'The Lord said to my Lord: "Sit at my right hand until I put your enemies under your feet." '
⁃ Mark 12:36 David himself, speaking by the Holy Spirit, declared: " 'The Lord said to my Lord: "Sit at my right hand until I put your enemies under your feet." '
⁃ Luke 20:42 David himself declares in the Book of Psalms: " 'The Lord said to my Lord: "Sit at my right hand
⁃ Acts 2:34 For David did not ascend to heaven, and yet he said, " 'The Lord said to my Lord: "Sit at my right hand
⁃ Hebrews 1:13 To which of the angels did God ever say, "Sit at my right hand until I make your enemies a footstool for your feet" ?