In the Splashtop Business app, the highest resolution that Splashtop can support will be 1920x1080. In cases where monitor resolution sizes are larger such as 2560x1080, since we currently don't support anything larger than 1920x1080, it is recommended to use the Virtual Display Driver option as a workaround.
Please refer to the following links for Virtual Display Driver set up for both Mac and Windows.
Windows
Mac
Please Keep in mind that with an emulated virtual display, the session window will not have a resolution of 2560x1080, but if the window itself is small they can click View Options (eye button on the top of the toolbar while connected) > Zoom to Fit.
Windows:
Mac: