The Model Size PMI command (A) sets the text size of all PMI elements to display using the same font size as the currently active style. You can change the text size by editing the defined font style using the Styles button (B).

Model Size PMI means that when you zoom in or out of the model, the text size scales, too. If you want PMI text to display at a fixed size, use the Pixel Size PMI command, instead.