Welcome! Need more info. There are a couple of smaller models that work well (Joye 510 manual, KR808D-1 manual) but the battery life is short on smaller models and you cannot use the hotter low resistance (LR) atomizers or cartomizers (heat enhances hit). Then there are popular fat battery models (like Ego and Riva 510) with much longer charge and the option to use the hotter LR attys/cartos. And beyond that are mods of various shapes (like a box or screwdriver shape), some variable voltage, some with a juice bottle built in, and more.
What strength of cigs do you smoke, how many per day, what is the longest amount of time you are away from power every day, and how much do you want to start with a slim model?
And for reference, 100mah is about 1 hour of moderate vaping. And most people here use and recommend manual batteries (button) - safer from being damaged or killed by juice leaks, easier to get a good hit.
Joye 510 180mah for standard (about 100mm long w/ carto), or 280mah for the 510 mega xl (128mm long w/ carto). Shipped as a 3-piece, more pleasant as a 2 piece with blank cartomizers like Boge standard resistance 510 cartomizers.
KR808D-1 (Vapor4Life, SmokelessImage Volt, Bloogplanet) 180-220mah for the shorty (about 100mm long) w/ carto, 280-320mah for the standard size (about 120mm long w/ carto). Ships as a 2-piece with prefilled cartomizers available. A DSE901 atomizer w/ cart (or more conveniently with a drip tip if you have manual batteries) can be used to test juices.
Ego and Riva 510 - typically 650mah+ (mine is 750mah and batteries available up to 1300mah). Sold as a 3-piece atty+cartridges. More typically used with cartomizers and/or atomizer+drip tip. Tank atomizers and tanks also available. Can share standard resistance atomizers and cartomizers with slim 510, but not share chargers or LR low resistance attys/cartos.
All of these use eliquid and plenty of it. Most people vape 1.5-5ml per day. Tanks and LR items may use juice faster. Cartomizers, a combined cart and atomizer, are disposable and a popular way to vape (no need to clean atomizers, flavors stay separate). With thin light juices I can get at least a week's use out of a carto and they cost $2-3 apiece for standard stuffing-type cartos.