Your starting line si almost right,
Dim payee2 As String = cboAccounts.Text
Then use the Length property, it shows you the number of characters in the string (do not use the MeasureString functions - they are used to get the number of pixels for the string with a given font etc.).
If it's too long, make use of the SubString function:
txtPayAgainst.Text = payee2.Substring(0, 10)
txtpayee2.Text = payee2.Substring(10)