To improve your control of the image resolution for WMS map images, you can set the GetMap request pixel limits both globally and locally. In this case, more pixels means higher resolution, but it also means slower access time. Because WMS servers are not required to specify pixel width and height limits in their capabilities documents, you will have to experiment with the pixel limits of a particular server to get the highest resolution you can. When you send a request that exceeds the server's pixel limit, the server usually issues an error indicating the maximum allowable image size. This will help you adjust the pixel limits for the server.
You can set a Web Services preference to set the pixel limits for all WMS images and you can override these limits by setting the per-server limits on an individual image basis. These options allow you to control the resolution of the WMS maps and limit the size of the maps you receive.