Hello Shadee,
Region section in the export orthophoto dialog represents the rectangle orthophoto will be generated for.
When you press Estimate button default values will be shown. You can reduce the size of the area by changing coordinates of the opposite corners of the region rectangle.
Pixel size is the resolution of the exported orthophoto and you can reduce it if the computer doesn't have enough memory to export the orthophoto as a single block, alternatively you can use Split in Blocks feature.
In case you still have problems with the export please provide the screenshot of the Export Orthophoto dialog with the default values for Pixel size, Boundaries and Coordinate system.