Using ACE/ULEIS and WIND/STEP >40 keV ion measurements, we have calculated the probability of simultaneously observing ion events upstream of the earth's bow shock as a function of the lateral separation (in YGSE and ZGSE) of the two spacecraft, thus providing a quantitative measurement of the spatial size of upstream ion events. Our survey consisted of 202 days in 1997 and 1998 when the ACE and WIND spacecraft were near the first Lagrangian point, approximately 200 RE upstream of the earth. We find that for lateral separations of up to 70 RE, the coincidence rate of events at the two spacecraft remains high and conclude that the source region of the upstream ions is large when compared to the size of the bow shock. ¿ 2000 American Geophysical Union