Well I did look up your phone specs here
http://www.gsmarena.com/samsung_galaxy_core_prime-6716.php
As it says it supports up to 64GB SD storage.
Getting an SD card varies from what you will use it for. I use the largest capacity my phone can support because I need to take a lot of Pictures and HD videos for my job.
So it's all up to what you'll be using your device for. My advice get a large capacity, good quality and high transfer speed card so that you'll have room on your device to do everything you might think of. But if you're in a budget a 16Gb card would suffice.
Also take note of the user manual of your device if it requires a minimum Class of SD card. If it says minimum class 4 SD card you should get a class 4, 6 or 10 card. As a class 2 card with not be supported.
As guide for how the classes work:
CLASS 2 - a guaranteed minimum write speed of 2MB/s or higher*
CLASS 4 - a guaranteed minimum write speed of 4MB/s or higher*
CLASS 6 - a guaranteed minimum write speed of 6MB/s or higher*
CLASS 10 - a guaranteed minimum write speed of 10MB/s or higher*
* The minimum speed capability depends on the manufacturer of the host device and model.
Hope that helps
