Hello Jove,
(I was in the process of posting this, and then saw alumbagh's reply, which is correct. Just be aware that there are other limitations when using 'Paint'.)
You need to get the 'Text Toolbar' displayed. After you click the 'A' icon/button and create a textbox, then click the 'View' menu item and enable (click) the 'Text Toolbar' menu item -- be sure to do this while the textbox is still active/selected. You should then be able to change the font/size of the text in that textbox.
Once you have enabled the 'View, Text Toolbar', it should pop-up automatically the next time you click the 'A' button/icon and then click in the image to create another textbox.
Also note that, as stated in the Help:
"You can enter text in a text box only once. When you click outside the box, the text is converted to a graphic image and you cannot make it active again."
What that means is that you can only change the font/size of the text when you are in the process of creating/editing/manipulating the textbox "the first time" -- just after you create it (i.e., while it is still "active"/highlighted/bordered). Once you click outside the border of that textbox, you can't change anything in that textbox (using the 'A' icon/button).