If you require another size simcard and keep same giffgaff number then easiest option would be to walk into any market stall phone shop and have your simcard cut down to size so it fits the new phone. these shops also sell simcard adapters if you required a bigger simcard later.
You can perform a free simswap, simswap is a system which is used to replace a simcard with another one, you will keep your existing phone number and credits or goodybags, please note that other data (e.g. contacts & texts) are not transferred, so make sure to save them before you start:
Order free from this link once you are logged into your account:
Pick giffgaff simcard from most corner shops, local market stall phone shops
If you don't have an unactivated SIM, you can order a new SIM here.
To perform a SIM Swap:
If you have an active PAC code on your account then the link above will not work for you. Instead follow this link to perform a SIM swap and proceed from point 3.
If you have lost the plastic card on which the activation code was printed then simply click 'Can't find your activation code?' and enter the 13 or 19 digit SIM Serial Number instead.
The SIM Serial Number is printed on your SIM card just by the 3Ggg1 for 3G SIM cards or 4Ggg4 for 4G SIM cards. If you have a Nano SIM, enter 894411 followed by the 13 digit SIM Serial Number found on your SIM.