Size of RichTextBox

I'm not sure what it is that you are asking for in 1.. All Controls have a Size property, if that is what you want. I suspect, however, that you mean the size of the area available for text.

The only way that I can think of is to use P/Invoke with the EM_GETRECT Message[^]. I've not tried it, so no guarantees.

As far as 2. is concerned, it depends on whether you want a percentage of the number of lines, the number of words or the number of characters.

Lines
The richtextbox has a Lines property and MyRTB.Lines.Length will give you the value.

Words
Here is a WordCount Method.

private int GetWordCount()
{
    // Add space to end of text to ensure the last word is counted.
    MatchCollection wordColl = Regex.Matches(richTextBox1.Text, @"[\W]+");
    return wordColl.Count;
}

Characters
myRTB.TextLength may be enough for what you need.

In all cases a quick bit of math will get your answer.

For deleting the text there are many methods but one of the easiest is to use the myRTB.Text.Remove(int)[^] method, or one of its' overrides.