Larger sproket on a single speed crankset? I recently bought an urban bike that comes with a single gear crankset with a 42 tooth sprocket and an 8 speed rear with 12-36 teeth. I would like to swap out the front sprocket to a larger one to gain more top end speed. I'm thinking 48-53 teeth depending on what I find. I wonder how I would go about doing this. Can I just screw off the 42 and attach a 48 to the existing crankset and then lengthen the chain a bit? Or would I need to purchase a new crankset? Would I need to adjust the rear derailleur if I run a larger front sprocket?
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.