Here's a way to enlarge bibtex that works with teTeX 1.0.  Change the number
5000 in the following lines in texk/web2c/bibtex.ch:

    @x [still 14]
    @!buf_size=1000; {maximum number of characters in an input line (or string)}
    @y
    @!buf_size=5000; {maximum number of characters in an input line (or string)}
    @z

to a bigger number, like 12000:

    @x [still 14]
    @!buf_size=1000; {maximum number of characters in an input line (or string)}
    @y
    @!buf_size=12000; {maximum number of characters in an input line (or string)}
    @z

and recompile.