What is the smallest Turing machine where it is unknown if it halts or not?
The largest Turing machines for which the halting problem is decidable are: 

$TM(2,3), TM(2,2), TM(3,2)$ (where $TM(k,l)$ is the set of Turing machines with $k$ states and $l$ symbols).

The decidability of $TM(2,4)$ and $TM(3,3)$ is on the boundary and it is difficult to settle because it depends on the Collatz conjecture which is an open problem.

See also my answer on cstheory about Collatz-like Turing machines and "Small Turing machines and generalized busy beaver competition" by P. Michel (2004) (in which it is conjectured that $TM(4,2)$ is also decidable).

Kaveh's comment and Mohammad's answer are correct, so for a formal definition of the standard/non-standard Turing machines used in this kind of results see Turlough Neary and Damien Woods works on small universal Turing machines, e.g. The complexity of small universal Turing machines: a survey (Rule 110 TMs are weakly universal).