Hi Nat,
all depends on the C5 configuration.
If you have the original battery cover with the key unit,and you want to retain the original configuration. then you are limited to a given size plus the plus pole needs to be located on the left (with the poles nearest to you), you will also need special terminal post adapters. If this the case, just let us know, and we can lead you on from there.
On the other hand if you just have the normal terminal connectors you can use a starter or a leisure battery, but watch out concerning the battery length, the space under the seat is very limited. Do a search in the forum, there's a lot of information listed.
The C5, and other electric vehicles, are designed to run on deep charge batteries (leisure), as deep charging a starter battery can lead to damage. Lots of the C5 owners are using the starter batteries, so it's up to you as to which type you use. Just a tip, the starter batteries are the cheapest.