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Again and again we see St. Thomas circle back to the most profound "mountain top" judgment that he gave back in Q. 4, God is ipsum esse per se subsistens. From this, as we deduced the perfection and simplicity of God, so also do we deduce the infinity of God.
In order to understand this, we must understand something central of Thomistic metaphysics, i.e., the relationship between perfecting and perfected. For St. Thomas, as we briefly covered in our comments throughout Q. 3, there are a series of relationships between that which is metaphysically perfect and that which is metaphysically perfected.