I have fished there a few times in my 16' mod V, which is a beater aluminum boat. I'd say 16' is about the biggest you could use and a jon boat would be great. I would not take a fiber glass boat or a boat I was worried about scratching out there. mostly its very shallow, alot of huge carp and cats. I had some good luck for bass between the vistor center east to the damn at the brides. there is tubes and at either side of the tubes got some good LMB 18-22". the locals look at you like your off your rocker when you tell them your after bass and your thowing them back. It is very peaceful and not crowded. never even seen another boat there just a couple people fishing from shore for cats. Check it out, its different. there is a campsite on Rt 6 w/ a ramp and you can pull your boat up on the bank and pitch a tent. Not many places in N. IL you can do that.
The Henn. is full of bass, you may think i'm crazy but I fish out of a 21' bass boat in the canal with ease. I fish mostly at the ramp right of of RT34 think it's lock 21? It's close to home and without starting the big motor it's cheap. I go there mostly on afternoons to pratice flippin and do some frog fishing. The biggest i've caught was 5-1/2lbs and i've seen days when you would get a blow up on topwater every 10 casts or so!