The EnableStream method allows you to specify the resolution for each stream. For a more robust way (ie, checking which resolutions the camera supports first then picking one of them), look through the RawStreams sample to see how the streams are enabled there.
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If you are trying to do this on Euclid, you have to change the color_height and color_width parameters in this launch file: https://github.com/IntelEuclid/realsense_camera/blob/master/launch/lr200m_nodelet_default.launch. The file can be edited directly or using the Euclid browser interface. The easiest way to do this using the Euclid browser interface is described in the attached document.
I hope you find this helpful.
Have a nice day.