2-letter words:
* AX
3-letter words:
* FIX
* TAX
* MIX
4-letter words:
* BOX
* FOX
* LAX
* SEX
* WAX
5-letter words:
* BIX
* EXES
* OXEN
* SOX
* TEXT
6-letter words:
* AXIOM
* ANNEX
* BOXED
* COAX
* FOXES
* OXO
* FAXED
* FIXED
* FLAX
* LUXE
* MIXED
* OXIDE
* TAXED
7-letter words:
* LAXITY
* MIXING
* OXIDIZE
8-letter words:
* ANNEXED
* COMPLEX
* FOXHOUND
9-letter words:
* TAXIDERMY
10-letter words:
* EXCITATION
11-letter words:
* TAXONOMIST
12-letter words:
* PHOTOSYNTHESIS
14-letter words:
* HEXADECIMAL
This is not an exhaustive list, but it gives you a good starting point!