The "area" that you want to minimize is the number of digits, d, times the base, b.
d is roughly equal to the log of the number represented, N, base b.
Substituting,
Take the derivative of the area with respect to b and find where the derivative is zero to find the minimum. Using the quotient rule,
I hope I got that right. Doing math on the internet is always dangerous.
Although the cost function has a multiplication of a base times the floor of the log of the value with respect to that base plus one, area is a misleading analogy to describe the applicability as any geometric dimensional value has to taken with respect to a basis. For a visual, (directional) linear scaling is more in line so to say.