I am thinking about upgrading my old batteries (they are only 40 W anyway) 40W is the highest wattage battery I have ever used so I don't really know how high I should go.
It would be ideal if it was not a fat battery (my current ones are about 1.25 in long and .75 in wide although it can be a bit bigger and I don't care how tall it is). It is also preferable that it looks cool.
Judging from the responses to your question, we're confused if you're asking about
internal battery mods or
external batteries for regulated mods. Knowing this would help us answer your questions.
Perhaps you yourself are confused by the terms
internal vs
external batteries?
Internal battery - seen in electronic mods where the battery can not be removed; charged by micro USB.
External battery - used in both electronic and mechanical mods. Battery can be charged by micro USB or more preferably removed to be charged in an external battery charger.
We choose external batteries based upon their
specifications; primarily the "amp rating" or "maximum continuous discharge rate" and secondarily the capacity or "mAh rating".
The battery with the highest mAh capacity is not necessarily the best battery for our purposes. And having said that, we don't necessarily need a battery with the highest amp rating (30 amp cdr) if we are not a high wattage vaper.
For example, my sweet spot is 30 watts. I can get the best performance from both a amp rating and capacity rating perspective with a 15 amp 3000mah battery. If I was a 75 watt vaper, I would get better performance with a 25 amp 2600 mah battery. The amp rating should always be your first priority, and the mah rating your second priority, when choosing which battery to use in a battery device. Which battery you end up choosing should depend upon the wattage setting you will likely use.
Battery Basics for Mods: The Definative Battery Guide for Vaping
Deeper Understanding of Mod Batteries Part 1
It is also preferable that it looks cool.
Really?
P.S. Is temperature control worth it?
IMHO, it is not worth it. I've tried it on a couple of different mods and found no benefit or advantage to it. TC seems to require some extra tweaking that wattage/power mode does not. However, most regulated mods today seem to include it. My advice would be to learn how to use wattage mode first; then if you wish to experiment with TC later then do so.