I don't know how big your safe is but the 12 inch rod keeps my safe around 50% humidity and about 70 degrees. My safe is a mid size one.I run a GoldenRod in my safe and keep a humidity gauge in it and my safes humidity never goes above 50% most of the time it is to low , so I should probably get a smaller GoldenRod . It might raise the temperature by 2 or 3 degrees . Right now the humidity in the room is 50% and temperature is 68 degrees . In my safe the humidity is 37% and 71 degrees . I have never had a problem using a GoldenRod and they guarantee against it .
That should do . I've read some place that 50% humidity is ideal.My safe is 36 x 60 x 24 , I have a 18" rod in mine . I think I will go to a 12" rod .