How can I automatically change the font size?

Is there a built-in function in iText that automatically reduces the font size to the maximum possible size, or do I have to implement this by myself?

Automatic font size is only possible in the context of AcroForm text fields. When you define the font size of a text field as 0, then a font size is chosen that fits the rectangle. In the case of a fixed cell height in a table, you are responsible to make sure that the text fits.

If you're concerned about the height, please take a look at the FitTextInRectangle example:

BaseFont bf = BaseFont.createFont();
int textHeightInGlyphSpace = bf.getAscent(text) - bf.getDescent(text);
float fontSize = 1000f * fixedHeight / textHeightInGlyphSpace;

If you're concerned about the width, then you need to use the getWidthPoint() method :

BaseFont bf = BaseFont.createFont();
float width = bf.getWidthPoint("My text", myFontSize);

You'll need to make sure that width doesn't exceed the width of the cell. To achieve this, you'll need to adjust myFontSize.

See my answer to this question: How to choose the optimal size for a font?