could be an optical illusion but assuming the string is around 38mm[1 1/2"] the gap looks a bit more than 94x94mm ??
the important thing is maximum contact with the string so it can be as one with the string by default with any padding out and support boosting the now solid string and newel set up
once you remove timber above floor level it restricts the amount off support you can import as in iff you have 70mm on the string above floor level you will get far more than 50% extra support over say 45mm proud
now off course any fixings below floor lever will help vastly so iff floors and celings are being opened up for further access for fixings please ignore my rantings