Estimating software projects will never be my strong suit, but I've learned using numbers from the Fibonacci sequence to judge the size sets me off to a good start.
To estimate a task (in hours or days), I only use numbers from the Fibonacci sequence:
1, 2, 3, 5, 8, 13, 21, 34, 55…
The further along the Fibonacci sequence, the bigger the difference with the next number becomes. This aligns well with how we should estimate, because the bigger the task, the more unknowns there are.