I've checked sed on my fedora linux and I've found that sed has a maximum string length: 2^15 - 1. So, you can write regex with max length of N = 32767
sed -r 's/N{32767}//g' bigFile > newBigFile
Also, you can multiply this value, like this (e.g. multiply on 3):
sed -r 's/(NNN){32767}//g' bigFile > newBigFile
Even, you can play with regex rule without right border if it acceptable in your case:
sed -r 's/N{32767,}//g' bigFile > newBigFile