Larger sproket on a single speed crankset?
You'll have to wait until you get the bike &amp; measure the distance between the holes, using this method. Then you can order the appropriate size chainring. Such as those here.

Rather than trying to lengthen the chain, I would recommend you buy a new one as you will be much less likely to have a tight or bent link this way. 

You should not need to make any adjustments to the derailleur if you are only changing out the front chainring.