Use font tag, choose a standard font (you can go to W3Schools if you need) and then still inside the <> write size="" and then a number of your choice.
Or use the header tags like h1,h2, h3, h4...
Still, to make evrything more neat, I reccomend CSS bc it will alllow you to make much more if you like making things pretty and having the right position and such or more complex layouts.
EXAMPLES:
Red foxes are agile and cunning creatures known for their striking reddish-orange fur and bushy tails. They are highly adaptable, thriving in diverse habitats from forests to urban areas.Ravens are highly intelligent birds known for their striking black plumage and complex social behaviors.
You can see what I did here selecting the sentences and then right click, then Inspect Element or Ctrl+Shift+I.