The size of your caulker depends on the size of the job. The larger the area you have to cover, then the larger your caulker should be. If your job has a lot of small nooks and crannies then you may wish to use a smaller caulker or scraper for this.
Caulkers are measured by the width of their blade, and come in a variety of different sizes. The most common range is between 90-300mm.
Here are some common metric sizes and their (approximate) imperial conversions: