fully assembled, the 650mah echo is only 1-2mm shorter that the 650ego. It just feels smaller somehow and lighter without the manual switch. It will put out identical vapor to your 65mm just last longer and the bigger carts have more filler and tend to mute flavor slightly although the echos are the best I' ve tried Start to think in terms of mah. 65mm=180mah (about 2hrs use) 78mm=about 280mah (about 3 hrs use) xl and elite=380mah (about 4 hrs use) 650= about 7 hrs, etc......
If you want to try larger, first try the 78mm and get used to it. When you go from there, I'd recommend the elite, which adds width instead of length. Those 380mah skinny's will poke someone's eye out.
All of these
batteries run around 3.7-4.2v The (standard) ego only runs at 3.2-3.4v so if you wind up with a 510 style ego, you will have to venture into LR cartomizers. 2.0 (like the 510 boge 2.0) seems to hit that same sweet spot at a standard kr808 but LR carts shorten the run time of the battery.
(It's all about Volts vs ohms to equal watts- there are plenty of posts that go into detail here but that's it in a simplistic package for you)