Short words have less room for variation
In a four-letter search, one confirmed position describes a quarter of the word. In an eight-letter search, the same clue covers only one eighth. Longer words usually need more information before the result becomes manageable.
Fragments grow more useful in longer words
A two-letter beginning or ending can remove many longer candidates while preserving the structure you know. When the order of the letters is uncertain, keep them in the contains field instead.
Use counts as a practical guide
There is no ideal number of filters. Stop adding clues when the remaining list is small enough to review. A reliable list of twenty words is more useful than a list of two built from a doubtful assumption.