In order to resize or format text, just select the words and look for the given options. Different sizing alternatives are available within the text style options, just hit the drop-down button to find the options.
Basic formatting options like Bold, Italic, and Underline are also found here, as well as Emojis and Bullet/Numbering. You can pick a color from your current Palette for the selected text.
On the Design editor, you can rearrange text boxes in the slide manually and extend or shrink them. Keep in mind this won't affect the font size.
If you're having trouble with text formatting, try removing the current format first and set up the new format options you want.