There isn't a firm upper limit. But a big basket with extensions will still be supported on bars that are only 27" apart. In other words, the bigger the basket, the greater the overhangs. Basket also don't increase the weight capacity. You are still putting the load on the same 4 mounting pads. There have been a few reports of the welds on this pads breaking, most likely due to excess loads, especially loads that twist or lift. Weight also reduces the rollover stability of the car, though I have not read of any accidents attributed to this.
This basket has about the same dimensions as a Yakima Loadwarrior, but at less than half the price, and less weight. It probably is not as stiff as the Loadwarrior, expecially with the added extensions.
I can't speak to the basket you are interested in but I have the Yakima LoadWarrior and love it. It is extremely well made and I have no concerns that it will last forever and never fail me. I mostly use it on my wife's car (Volvo XC90) and have an extension for it - the regular basket size is 44"L x 39"W and the extension adds 18" to the length. They also make a MegaWarrior basket which will fit the E - it is 52"L x 48"W and they have a 22" extension that is available.
My only issue with it on the Element is that the rack attachments on the roof of the car are fairly close together, so it extends out quite a bit in front/behind the racks if you use the extension. This is not necessarily a problem as you're not supposed to heavily weigh them down anyway but I'm a bit more careful about how I pack the load, how much I put up there, and I don't step on the basket outside of the racks when I'm loading it. Since the E is a wide car and uses wider crossbars than most cars, it would fit the MegaWarrior and I think perhaps a MegaWarrior with no extension might fit the car and racks better than a LoadWarrior with an extension (i.e. wider and shorter, rather than longer and narrower) and the usable area would probably be similar.