The image that non-WWE viewers see in the first ten seconds they stop on Spike is extremely important. 98% of viewers are going to see a WWE ring, a WWE stage, a WWE screen, and the announcers sitting where Vince McMahon puts his announcers-- 98% of viewers see a WWE show and don't give it two ****s after that ten seconds.
A 24ft square ring would be a vast improvement over the 18ft one, but it would still be more to the company's benefit to define its image from the WWE's. A hexagon ring would please the core TNA audience while making the product safe for non-viewers who hate the WWE.
The WWE has created a nasty stigma toward wrestling and TNA has to overcome that by boldly saying they are not the WWE. For most Americans WWE = wrestling.
This is what 24ft looks like.