Procedure

Select convenient length
Select max temp - such as 550C = 1000F
Match gauge to proper voltage drop of 120V - ie, observe parameters that produce 120V drop - those will be the working parameters since 120V is fixed.
Find out max gauge that is within current limits (15A) and has correct voltage
Limit for 15A outlet is 1800W

Sample:

Remains within 15 amps
Shows correct voltage
Max temp - select it to match voltage - you determine max possible temp and judge whether it will burn out your system if you have temperature sensitive components
Note that Gauge 17 is the max fatness that stays within 1800W
Note: 22' of 18 ga gets us right at 120V, and 1560W. Good. But temperature of 1260F is a little high.

1000F Limit - 240V Case

12' of 26 ga draws 450 Watts, but goes higher than rated 1000F
This was acceptable, but burns out thermistors if placed directly on heater.
Point: the shorter the nichrome, the hotter it will get while remaining at the correct voltage
Keep conservative on the bed power,then. Use 14' of 26 ga for 380W and 1000F max at 120V.
For same power density on large bed (7W/sq in), would need 4000W of heat. Can't do that. Limit is 1800W. the question is how best to achieve it - what length of nichrome is required for a workable length, while keeping temperature lower rather than higher.

1800W limit
To cover bed - try 6 lines separated by 4" of space. This means 12' + 2 feet return or 14' of sleeve
Gauge 17, 28' long, gets max 1090 and 1570 watts
sleeve it in 14' of sleeve. Quite doable. needs total of 28 feet of sleeve.
Selection:

Shorten it by 2-4 feet to get max power and temperature.