David Haye standing at 5'4" is the smallest hw champion ever. Against Valuev he traded away 2 foot and nearly knocked him out, he also nearly knocked out top 3 ATG hw Wlad with a single punch. Thompson barely survived him and was never the same again.
Smallest as in lightest, then I think Bob is the lightest champ. Though he was as tall/ little bit taller than Tyson.
Same with Harry Greb. Greb was 5'8" ~ around 160 lbs and actually beat a few HW champs. The OP asked who was the smallest, not the shortest, so Greb & Fitzsimmons are probably tied for 1st.
Speaking of Dwight Qawi, I was just reading a 2007 interview with him, right here on ESB, and he says straight out that he thinks Holyfield was juicing during their fight. Maybe you guys have already seen this, but if not, it's fairly convincing, and also a great interview all around: http://www.eastsideboxing.com/weblog/news.php?p=10932&more=1