You can take a look at this thread here on Gimp Chat.

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That's a good question.
To my knowledge,
this really hasn't been determined yet.
BRL makes it's own e-map when a gradient is selected.
From the 3 different size examples below.
You can see the maps that were generated are each about 308 pixels each in size.
Although these maps are the same size.
Some have more detail then others.
The amount of details in each map,
or lack of detail.
With the maps being basically the same size.
I can't explain.
I would conclude,
that if you were to make your own spherical environment map,
or e-map for short.
I wouldn't make them any smaller the 308x308.







I believe that unlike spherical environment map.
Flat e-maps should be roughly the same size as the image they'll be use on.
For instance,
if I had an image that was 800x400.
When using a flat e-map.
The flat e-map should be equal to or slightly larger in size then the image.
This doesn't seems to apply when using a spherical environment map.

I made this image using the flat e-map below.