http://www.theverge.com/2016/2/16/11018 ... rage-glass
Of course it will take many years for this technology to be at a point where data can be encoded affordably and read outside of a lab. But that will happen, either with this invention or with another one, waaaayyyy before the blocksize is hundreds of terabytes

So permanent backups for bitcoin are on the way. No size constraints here.