Thing to consider first maybe is, what kind of vape you want to build? What's the bowl size and material you're going for and how much herb can it hold? How far away from the heater is the bowl?
For a log vape for instance, that uses a tiny stainless steel bowl < 10mm, a cartridge heater or resistor in the 20ohm vicinity and with a surface area resulting from a mere 6mm x 20mm diameter set into a < 10mm steel tube, is already enough to heat up the air passing to about 230°C at 12V/7W and gives you huge clouds. Anything more powerful, will result in combustion.
Set that same tiny cartridge heater into a wider enclosure (maybe an 18mm female tapered glass joint, like in the splinter/iHeat) for heating up a bowl twice that size in diameter and all you'll get is a whiff of hot air but no clouds. You want a more powerful heater there with a wider surface area to get the same results.
Then there is the question of pre heat time and usage.
Given ideal conditions, the 20ohm/7W cartridge in a log vape needs 5-7 minutes to reach usable temps for vaping. Ok for a plug-in/desktop vape, but no use for a portable, where you want it going within seconds to maybe one minute tops and also won't have it running several hours, but only for minutes at a time. So a portable log vape following otherwise the same design with the tiny ss bowl and all, wouldn't be helped with a higher surface area heater there, but rather one of the same surface area, but of a lower resistance, capable of churning out more watts at the same or even lesser voltage, to speed up pre heat time and be ready to use (for my own DIY portable, I use a 3,66ohm/40W cartridge there, driving it at 10V, producing about 25W of power).
So an 'one size fits all' approach to the heater might be difficult in general. And just thinking of the size... 6" x 1" would be a really fat bugger indeed. Heater bigger, than me whole log, wood and all