The idea that the ion pod resides there dates back (at least) to 1978, published in StarFleet Assembly Manual Number 3:
http://www.trekbbs.com/showpost.php?p=3818898&postcount=21
I dare say that consistency with a fan publication from 1978 shows a great familiarity with this issue and Trek publications.
KIRK: We'll need somebody in the pod for readings.
http://tos.trekcore.com/hd/albums/1x20hd/courtmartialhd003.jpg
http://tos.trekcore.com/hd/albums/1x20hd/courtmartialhd049.jpg
How exactly would Finney fit into the ion pod as depicted in TOS-R? He'd get half his body inside easily, but would then get into in a fetal position? to cram the rest of his body in? And still take readings from the sensing devices (aka ion plates). You can see in the 2nd screenshot the two spaceguys moving the bulb in place for size comparison.
Picking that blinking light bulb as the ion pod doesn't show any familiarity with the episode's dialogue, IMO. Which is a shame since they had a CG model and could've really depicted a larger pod or opt to not show it at all.
About the same size:
http://science.howstuffworks.com/project-mercury2.htm
Just 74-inches in diameter.