you will have to create the graphics for the rich text control, parse the text you wanted to parse and get the size of that text.
var font = new System.Drawing.Font(this.Font, FontStyle.Regular);
Graphics graphics = richTextBox1.CreateGraphics();
SizeF textSize = graphics.MeasureString("A", font);
float width = textSize.Width;
This will be the width of the text you are parsing, if you parse a word, it would be the width of the word and so on.
You must bear in mind that this single Font parameters into account, if the text is mixture of Font parameters then it should be added in a loop of fonts and corresponding texts.