When you create a PowToon text, you can only add a style to an entire text string. It is not currently possible, for example, to have a five-letter word where the third letter is bold and larger than the other characters.
However, what you can do is time a series of textboxes to appear at the same time, giving the same effect.
There are no limits to what you can create; however, here are a few examples of this technique being used.
In ancient texts and religious calligraphy, you will often see that the first letter of a sentence is a larger, and very often ornate, image. You can use a mix of PowToon Studio fonts to achieve this same effect, and it can look wonderful, as shown here: