UtaForth – 303-byte 16-bit Forth in pure Netwide Assembler (github.com)

Although it requires DOS, in contrast to other tiny 16-bit x86 Forths such as MilliForth[0] and SectorForth[1], this is the smallest of such Forths that I know of.

0. https://github.com/fuzzballcat/milliForth/ 1. https://github.com/cesarblum/sectorforth/