How to select for 1-4 carbon sugar bond in Chemical Glycosylation?
You have to protect the OH groups you don't want to react with protecting groups.

In this case you could install a benzylidene acetale between the 4 and 6 OH, then protect all the other OH and then do selective reductive opening of the benzylidene so your benzyl group will be on position 6 and you got a free OH.
That said: it's quite expensive, labor intensive and has very bad atom economy. And you might run into problems with deprotection once you got polymer structures, so that's not a feasible option for synthesis of cellulose.

I don't think it's possible to optimize a synthesis without protecting groups, so the best way would be doing it enzymatically, which could be very expensive too.