Not all stores carry all the sizes. Remove the lock and assertain whether its an off set or equal. (If one side is bigger than the other). If equal measure full length. Back patio slidings are usually 30:30. Timber munster doors are usually 30:40. uPVC doors come in all shapes and sizes.
If you are replacing the locks, Id recommend anti snap cylinders. Two types available - mul-t-lock, and cisa. Google them for more information.
As far as colors are concerned, brass, satin or nickel plated.
Is the lock itself broken or just the cylinder [where the key goes in] A new cylinder would cost around €10, but a new lock is a lot more and there are many different types of locks