Any one rooflight design will come in many different sizes, often ranging from very small up to a large surface area. However, outside of these standard sizes, it may be hard to find custom rooflight sizes. If you are buying an in-stock rooflight, then it is going to be of a standard size, so you will need to see if this will suit your project.
If you do need a bigger rooflight, some will be able to be fitted together end to end to create a larger surface area. There is also the possibility of having a bespoke rooflight design. This is where the rooflight will be made to suit you, to your measurements, so you can have almost any size you like. This will still need to be within the parameters of what is possible for the specific type of rooflight and the technology behind it, so if you have a very awkwardly sized space, you may still struggle a little.