How to find how much text fits in a textbox without scrolling

I have a relatively large text. I need to add a certain amount of that text to a textbox so that it can be visible without scrolling, then add the rest of the text to another textbox, and then another one -.-.-. > Loop through the text, generating as many text fields as needed.

My problem is that I don't know how to find out which part of the text fits in each text box. So far, the only thing I have been able to do is assign a fixed number of characters that fit into the page. But that wouldn't be the case for different screen resolutions. Is there a way, trick, or workaround that I can use to figure out how much of the text can fit in a text box with fixed font and fonts, but relative width and height?

int TextLength = 1000, PageStart = 0;

List<TextBox> Pages = new List<TextBox>();  
while (PageStart < TextLength) 
{
     TextBox p = new TextBox();
      if (PageStart + PageLength < TextLength)
      {
             p.PageText = Text.Substring(PageStart, PageLength);
             PageStart += PageLength;
             Pages.Add(p);
      }
      else
      {
             PageLength = TextLength - PageStart;
             p.PageText = Text.Substring(PageStart, PageLength);
             Pages.Add(p);
             break;
       }                      
  }