Have you looked at ENVI's RESIZE_DOIT routine? This allows you to resize the pixels to be larger, thus making your image smaller. If you only want to process a subset of data, then you can pass in the dimensions you want to process using the DIMS keyword which is present in most ENVI routines. Then you can output the data to an ascii file using ENVI_OUTPUT_TO_EXTERNAL_FORMAT with the /ASCII keyword. If you specifically want to write to CSV then use IDL's WRITE_CSV routine.

Thank you for your help. I've played around with the resize tool, but I'm looking more into cropping the image rather than scaling it. Manually, the procedure is located under ENVI's Basic Tools> Resize Data (Spatial/Spectral)>Spatial Subset>Map. Unfortunately, I am unable to find how to get the code for this exact process.

As mentioned above, when using ENVI routines, you specify a spatial subset using the DIMS keyword. You should find a DIMS keyword for most ENVI routines. If you don't plan to do any further processing in your program, then you could do something like open the original ENVI file using ENVI_OPEN_FILE, then write a subset of that file using ENVI_OUTPUT_TO_EXTERNAL_FORMAT. You would want to set the /ENVI keyword, to indicate that the output file should be in ENVI format. And you will use the DIMS keyword to specify the spatial subset. - Peg

Thank you Peg, Will I be able to use the UTM coordinates in the DIMS keyword? All examples I've seen use x,y. Since the files I am processing are LANDSAT images, it's important that I can specify the subset using a geo-referenced (lat/long) coordinate system so that ENVI clips out the same area each time.

Not directly. You will need to figure out which image coordinates correspond to the UTM coordinates first. You can use the routine ENVI_CONVERT_FILE_COORDINATES for that. - Peg