In general, he was limited to an object of his own mass, or less, as a Changeling, via his Morphogenic Matrix:
From Memory Alpha,
A morphogenic matrix is the cellular and quantum structure of a changeling, which allows them to assume various shapes or forms. It also allows Changelings to shunt some of their mass into another dimension, thereby allowing them to morph into objects with a much smaller mass than themselves.
Note; the limit is equivalent MASS, not size, so a change to something like fog will permit a much larger area to be covered; in theory, he could become a really large balloon, for example.
As far as I can see, no specific limit on the amount he could shrink himself has been shown.
You may also find these worth looking at:
The Changeling Page