You noticed probably yourself that the 2-D barcode turns gray when you overload it.
As the barcodes zip the data it is hard to predict how many bytes of raw data fit in.
If you are precautious you fill every field with as many letters as possible. Constantly mix small/big caps, numbers, other letters. If your barcode still holds the input, then you can be pretty sure that you haven't overstreched it.
Your users usually will not write such a mess into your form. Therefore it is quite ok to deliver forms that might be overloaded - but not with real data. Your in the field of calculated guesses here.