How can I read large QRs in Android?
QR codes have limitations on the amount of data that can be stored within them. If you are hitting these limitations then you may need to find workarounds such as making the QR code text a hyperlink to the long text you wish to display.

For more information on the limitations of QR codes, see the "Storage" section of the Wikipedia article and its citations.

This citation in particular will be very useful to you. Look at version 40 of the standard in that table, as it has the highest capacity and will give you the absolute length limits.