On the principle that a picture is worth a thousand words:
As Kent indicated, the Mega eGO falls halfway between the two in length.
As for the Riva, at 750 mAh, it competes favorably with the original eGO/Tornado. I don't know its precise dimensions, but it appears to be about the same size as the original eGO and EastMall claims "all parts are interchangeable" with the eGO. It is less expensive and both EastMall and NHaler state that it has "an improved manual button."
I've read/seen some
positive reviews of the Riva (as I have, of course, of the eGO). Long term quality and reliability: ?
The best price I've seen is from VaporJunction ($60 + $7 shipping), although they currently are sold out.
As for the "or ?" in your question, there are the Hello 016 and Absolute MaxX:
- The MaxX (about the same size as the original eGO) had been claimed to be 4.2V, however, I've read that it is nowhere near that, probably about the same as the eGO's 3.4V ... and that claim has been dropped from the Absolutely Ecigs website. And it is the same 650 mAh as the original eGO.
- The Hello 016 is 130mm long and 16.5mm diameter, i.e., basically the same length as the new mega eGO, but 1.5mm fatter. Its claim of 1300 mAh may be somewhat inflated, but is certainly up there in mega eGO and Tornado range ~1000 mAh. Although no claims about its voltage are made, I've read that it is in the 3.7V ballpark (3.9-4.0V fully charged), apparently the highest of the "Fat Batts" (as I call these group). Its downside is the proprietary integrated cone-atomizer ("conemizer"?) it uses. All other 510 attys will work on it, but without any cone.
Its price is greater than the Riva, but comparable to the original eGO: $50 (from NHaler, which calls it the "Hello, Good-Buy") for a one-battery kit plus $25 for a second battery = $75. But for a high mAh (and higher voltage) Fat Batt, it's probably the cheapest way to go, since the new mega eGO batts are not (yet) sold as part of a kit. [You'd have to buy a regular eGO kit with two 650 mAh batts and separate mega batt (currently only available from Heaven Gifts).]
The Hello's build quality probably is not quite as good as the eGO's, but I've read some positive reviews, e.g.,
ab357's review and thread and
this long thread.
Personal Assessment: The
Riva seems to be the best bang for the buck: $67 to ~$75 for a 750 mAh, two-battery starter kit ... and my
subjective opinion is that it is the most attractive of the group (but then, like Goldfinger, I love gold

).