How to build Forth for embedded devices
You might want to check out pForth, which is a portable implementation in C.  The PIC16F877A has 14K of Flash, and only 368 bytes of RAM so there is no way pForth is going to fit.  The PIC16F877A also has a 16-deep hardware stack used only for return addresses, and no parameter stack.  These restrictions would have probably ruled out using a PIC16 for pForth anyway, even if you could get one with enough Flash and RAM.

Based on gbarry's answer, it looks like you're going to need a much bigger microcontroller for pForth.  You might as well skip the PIC18 (which has a maximum program size of 128K), and also the PIC24 (which also has a max of 128K) and go with a PIC32.  The PIC32MX170F256B has 256K of Flash and 64K of RAM, and costs under $4.  It comes in a 28-pin DIP instead of a 40-pin one though.

You also might want to check out the following books:

Programming A Problem Oriented Language: Forth - how the internals work by Charles H. Moore (the original creator of Forth)

This book is only available as a Kindle book, but you can get a free reader for the PC and most portable devices like the iPhone, iPad and Android equivalents. 

Forth Application Techniques: Course Notebook, 5th Edition by Elizabeth D. Rather (who was a co-founder of Forth, Inc.)