Shelly, I am not really good at being descriptive, but I shall try to help. The small and medium DAV's are almost exactly the size of a 3.5 ml tank system... meaning the tanks fit really flush on them. So just imagine the tank extended down the entire length of the battery to get a feel for the size. The XL DAV fits pretty well flush with the 6 ml tank, so is a good size bigger. In length, the small and medium DAV's are just slightly longer than a regular titan battery. While the XL DAV is just slightly shorter than the XL titan. Either one would easily fit in a cup holder.
I have never really timed how long they last battery wise, but I am a very heavy vaper, and I can get through most the day with 2 of the smaller DAV's and pretty much all day on the XL DAV (turned all the way up to 4.8). Of course the DC tank at higher volts will lesson your battery time to a certain extent.
I have the DAV's, auto Zeus and dual mode Zeus and if I had to pick a favorite, it would be difficult. I love the DAV's because they are adjustable and I can change them up for whatever tank carto I have on it. However, I usually find myself using the 1.5 dc in all my tanks, but some juices do better at different volts. I love the Zeus with the smileos....... because they are so made to be together. The voltage is set at 5 and that may be too much for certain juices with a 1.5 dc carto...... but the smileos can handle it because the are lower resistance and sc. I honestly have not tried my Zeus with a tank, so I can't really speculate on how I would like it. The advantage of the Zeus is the faster charging time and the fact that you can charge and vape at the same time if necessary.
I know someone will be along to describe the differences so much better (and maybe provide pictures) to help you out.... but I would say you won't go wrong with a DAV! You might want to try the smaller one to start and then work up if you need.