Consider decision problems stated in some “reasonable” formal language. Let's say formulae in higher-order Peano arithmetic with one free variable as a frame of reference, but I'm equally interested in other models of computation: Diophantine equations, word problems from rewriting rules using Turing machines, etc. An answer expressed in any classical formalization would be fine, though if you know how much the choice of formalization influences the answer, that would also be interesting.
Given the length $N$ of the statement of a decision problem, we can define the number $D(N)$ of decidable statements of length $N$ and the number $U(N)$ of undecidable statements of length $N$.
What is known about the relative growth of $U(N)$ and $D(N)$? In other words, if I take a well-formed decision problem at random, what is the probability of its being decidable for a given statement length?
Inspired by this question which asks whether “most problems and algorithms [are] decidable”. Well, if you don't filter by interest, are they?