Ok, that’s good.
What you want to do is select text (you can use shift+arrow keys, or click+drag), and then go to the text properties docker. There select the tab “character” to only change for the current selection, then go to font size, and press the up/down arrows to change the size.. To change over the whole paragraph, use the paragraph tab, for example, you can set the font by going to the font-family dropdown and selecting a font there.
There’s a manual page that gives an overview of some of the basics of the new text tool: Working with Text — Krita Manual 5.3.0 documentation