Cody,
There are several things to consider. Size is a big one, the other is battery charge. They kind of go hand in hand, but you will also need to decide what else you want it to do other than just be a VV. What I would consider an entry level MOD, something that does more just adjust voltage (like a twist), would be a VAMO V5. I picked up a Vamo V5 because I already, like most vets, have too many batts already and one more wouldn't hurt

Anyway, the vamo v5 is one of the cheaper VV/MOD batts with plenty of features like: VV, VW, ohm meter, batt charge remaining, etc. The nice thing is that you can use either 1x 18650, 2x 18350, or 1x 18350 batteries. You remove part of the batt tube and just use 1x 18350 and it is much shorter, but you have less vape time (just carry a second batt with you). You could get a v1, v2, or v3 that are a bit smaller, but have slightly different features. I like the v5 better, but it is a beast.
There are plenty of batts out there that might be better/smaller/sexier, but for the price point of $30-40 is pretty good with all of these features, and is very competitive. The VAMO is a 510 thread. Look at some reviews on youtube. You will get a better idea of the size and function for this or any batt for that matter.
Once I just wanted a cheap VV with plenty of mAh and since I use a meter to check ohms, I felt I didn't need a batt with that function, so I picked up an ego X6. Not bad, not too big but plenty of vape time between charges, and cheap ($12). It is much simpler than the twist in that it is not infinitely variable from 3.2-4.5 like the twist (X6 only has 4 settings), but they were in the ranges I usually use and was a 1300mAh batt, fatter but not longer, so more portable.
That was just to demonstrate that are different batts for different applications and needs. Figure out what you want it to do, how big, and then you will pick a better battery. Remember, there is no one perfect battery for everything, so you may end up buying several different ones over time.
Last note: almost everything in vaping is an individual personal preference; your mileage may vary, depending on your own needs.
Saludos