If your current giffgaff sim card doesn't fit your new phone, then you need to order a new sim card and do a Sim Swap so that your current mobile number is transferred over to the new sim card . Any credit or active goodybag/gigabag should transfer over to the new number once the Sim Swap process is complete.
GENERAL INFO RE: SIM SWAP:
1) If you want to transfer your giffgaff number to a different giffgaff sim card, then that's when you need to use a new sim card (order a new sim card if necessary) - and do a Sim Swap.
2) The sim card you want to transfer your current number to needs to be LESS than 6 months old.
3) The sim swap can take up to 24 hours to be completed.
4) You can only do a sim swap between 4:30am and 9:30pm.
5) Once the sim swap has been completed, any credit/goodybag/gigabag connected to the original sim card should be transferred over to the new sim card.
6) Make sure you back up any data you have on your original sim card (like contacts, for example), BEFORE you do the sim swap.
7) Sim cards being sent out by giffgaff now should be tri-sims,.meaning they are 3-in-1. There is a nano sim card inside a micro sim card, which is inside a standard sim card. If the standard sim card is too big for your current device, then you can just push out the micro or nano sim.
Website link to order a new giffgaff sim card of the CORRECT size on giffgaff website: http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084