A given length of wire will have a certain resistance. The thicker the wire, the lower the resistance. Coiling the wire allows this length of wire to fit inside your atomizer.
Eg.
Target resistance ~ 1 ohm
26AWG wire total length 89mm, wrapped 8 times with an internal diameter of 2.5mm
(If you made the internal diameter of the wraps 3mm, then only 7 wraps would be required.)
Target resistance ~ 1 ohm
24AWG wire total length 141mm, wrapped 12 times with an internal diameter of 2.5mm
If you are aiming for a lower target resistance then you will need to use a thicker wire.
If you are aiming for a higher target resistance then you will need to use a thinner wire.
www.steam-engine.org/ is your friend.