I was thinking more of the smoke being blinding, so say to 30 or 50 feet to get the vibe of The Mist*, and sickened within 10 feet as it is an emanation of the negative energy created while the victims burn.
* This also hides the nature of the wicker man and forces the players to guess what might be in the cloud. That loses some of the visceral nature of the creature as the characters can't see people being burned but makes up for it in other ways. Darkness, clouds and fogs are woefully under used IMO.
Until now I have never had much interest in using one. The similar druid constructs in the Monsternomicons for Iron Kingdoms are more my style (menhirs that stand up as wood and stone giants that use various magical attacks).
But now I too want to use a wicker man or a reskinned as a mobile giant's oven meant to catch and cook little people.