buffer size limits

I've been reading nested structure data very quickly
using:

(with-open-file (from-stream "foo.bar" :direction :input)
    (setf file-line (read-line from-stream))
    (setf struct-complex (read-from-string file-line)))

which works great up to about 50K of data,
but then around 80K the file-line read is shorter than
the actual file,  clipping the end of the structure values.

Is there any way to increase the read buffer size
so larger files can be read in fully?

Lawrence Au
Q-Phrase LLC