Many of Greg’s images have been offered in larger sizes as Limited Editions (meaning they are signed & numbered, and only a certain amount are ever printed). In order to protect the integrity and value of those editions, we don’t offer open edition prints (meaning they are non-limited, and can be produced forever without limitation) in sizes that are close to or larger than the Limited Edition sizes.
It’s possible that the Limited Editions of the image you’re interested are still available. Here is a collection of Limited Editions that are still available: http://www.gregolsenart.org/
We do have several images that were never offered as Limited Editions, which means we can print custom sizes to fit your needs. Let me know what images you’re interested in and I’ll let you know what you’re options are.