When building the GLT, ENVI will estimate the pixel size by the input latitude/longitude bands. It is during the building of the GLT that you have the option to use the default pixel size or set it yourself in the dialog. The ENVI_GLT_DOIT also has a pixel size keyword you can use to set the pixel size of the output GLT, or you can choose to use the pixel size ENVI estimates. So the resulting GLT already has a pixel size before you apply the GLT to the image. You could query the GLT for it's pixel size if you do not know it. There is no routine to provide the estimated pixel size of the GLT before the GLT is created. There is an example ENVI_GEOREF_FROM_GLT_DOIT example that shows how to call both of these routines.

Yes, Ok; I mis-spoke. Basically, the question then is: before creating the GLT, is there a way to get the "default selected pixel size" programtically? I understand how to input the pixel size to the ENVI_GEOREF_FROM_GLT_DOIT, and I understand how to just use the default. My question really was if there is a way to know what the default is before creating the GLT? From what you say above, the answer is "No: there is no routine to provide the estimated pixel size of the GLT before the GLT is created". Thanks.

It seems you could calculate this from the extents of the image divided by the number of samples/lines.