1. A computer program product for sharing a region of a computer system screen, comprising: a non-transitory computer readable storage medium; program code, stored on the non-transitory computer readable storage medium, for selecting an application to generate the region on the screen of a first computer system to share with a second computer system in a sharing session, wherein the application configures the first computer system to share the region with the second computer system using a screen sharing tool, and wherein the region is defined by a user of the first computer system to dynamically share a selected portion of the screen by creating a pixel map of the region wherein the region is selectively defined using regular, simple shapes, including rectangles or circles, complex irregular shapes of a usable form for selecting a useable region of screen, and a set of regular and irregular shapes, and wherein the region further comprising one or more regions defined on the screen; program code, stored on the non-transitory computer readable storage medium, for assessing information on performance of the sharing session, including assessing performance of client devices; program code, stored on the non-transitory computer readable storage medium, for determining from the information on performance a minimum size of the region based on the assessment; and program code, stored on the non-transitory computer readable storage medium, for selecting the region to share based on the assessment and a designation by a user.