You can buy digital downloads at the sizes your photographer has made available. A digital download is simply a copy of the image file, sized in one of four ways.
A Web Size image is roughly 640x480 pixels in size, or 0.3 megapixels. This is great for web sharing on social sites like Facebook.
A 1Mpix image is roughly 1 megapixel in size. In English, that means a square 1 megapixel image would be about 1000 pixels wide and 1000 pixels high.
A 4Mpix image is 4 megapixels. This is a size that allows for quality printing at most reasonable sizes. If you want to print at really large poster sizes, you would probably want a larger image.
An Original image is the exact image that the photographer has put online. It is the original size of the image, identical in every way.
Note: Not all sizes are available for all images. Downloads are subject to a license agreement.