You could either calculate the width of a space, the width of the text, the width of the window, and pad the caption with spaces (which is the easiest thing to do), or owner-draw the form's border and caption bar by subclassing and capturing WM_NCPAINT.
If you want to do the first one, you need to use the Graphics.MeasureString method (use the Graphics object of the form, retrieved with Me.CreateGraphics()) to calculate the size of the text in the caption.