3 Letters:
* know
* ledge
* won
* owl
* ode
* dog
* log
* low
4 Letters:
* know
* edge
* wold
* gone
* hold
* lone
* lode
* down
* gown
* doze
5 Letters:
* ledge
* know
* knowe
* whole
* dower
* gowns
6 Letters:
* knowledge
7 Letters:
* knowledge
This is not an exhaustive list, and there may be more possibilities depending on your rules for word formation.