May I suggest the use of Binary Lambda Calculus [1] for writing programs and measuring their size in bits?
There are many BLC programs of only a few dozen bits, comparable in complexity to 5 state TMs that need nearly 50 bits to describe, whose halting behavior is unknown.
Beyond that, there are programs like this 215 bit one for computing Laver tables [2], whose halting behavior is related to existence of large cardinals. A counterexample to Goldbach's conjecture can be found with a 267 bit program.