Below is a method that I wrote to determine the maximum string length based on a width. It uses a form standard method, so it must be a method of a form.

LPARAMETERS pcTextString,pnLabelWidth
LOCAL lnNdx, lcDrive, lcTest, lnStart
lnStart = ATC(":\",pcTextString)
IF lnStart > 0
*-*	Trim path
	lnStart = lnStart + 1
	lnNdx   = 0
	lcTest  = pcTextString
	lcDrive = LEFT(pcTextString,lnStart) + ".."
	DO WHILE thisform.TextWidth(pcTextString) > pnLabelWidth
		lnNdx = lnNdx + 1
		DO WHILE SUBSTR(pcTextString,lnStart+lnNdx,1) != "\" .AND. lnStart+lnNdx <= LEN(pcTextString)
			lnNdx = lnNdx + 1
		ENDDO
		IF SUBSTR(pcTextString,lnStart+lnNdx,1) != "\"
			pcTextString = lcTest
			EXIT
		ENDIF
		lcTest = lcDrive + SUBSTR(pcTextString,lnStart+lnNdx)
		IF thisform.TextWidth(lcTest) <= pnLabelWidth
			pcTextString = lcTest
			EXIT
		ENDIF
	ENDDO
ELSE
*-*	Trim text description
	pcTextString = pcTextString
ENDIF
RETURN pcTextString

You can possibly adopt this to determine how wide you need for the form.