If asked to guess a number that a human chose that is between zero and what they say is "infinity", how would one go about assigning probabilities to both a) assign higher numbers lower probabilities on average than lower numbers and b) assign higher values to low complexity numbers than higher complexity ones?

For example, 3^^^3 is more likely than 3^^^3 - 42.

Is a) necessary so the area under the curve adds up to 1? Generally, what other things than a) and b) would be needed when guessing most humans' "random" number?