I'd like to upgrade my SRAM cassette, but confused by options I have this bike: http://www.cxmagazine.com/blue-bicycles-norcross-al-cyclocross-bike-review,  with a 48/34 chainring and 11-26t cassette. I'd like to add some lower gearing. 

SRAM offers at 11-32 cassette (https://www.sram.com/sram/road/products/pg-1050-cassette), and it's supposedly for Apex, but I've been told I'll need a new rear derailleur as well - is this true? And will the 11-32 cassette work with the compact crankset? Thank you!
The length of the cage on the bottom of your derailleur that holds the two small cogs is what decides the size of cassette you can use on your bike.

There are 2 numbers that matter, the max size and tooth capacity.

Max size is is the largest cog the derailleur is designed to clear and the capacity is the largest difference between the small-small and big-big gear ratios that it can take up the slack in.

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Since your bike came stock with a 11-26, I'm going to guess it has a short cage derailleur, the largest cog this can accept is 28.

SRAM road derailleurs come in short and medium cage, and have a max cog size of 28 &amp; 32 and a max capacity of 33 &amp; 37 respectively.

You may also need to change your chain, not a bad idea anyway with a new cassette.