I think, with Sectumsempra, that the spell will slash your opponent in the direction and placement of your wand.

We also know <spoilers> that Snape used it against one of the Death Eaters and missed, and it cut George's ear off instead...</spoilers> This suggests that it will cut whatever you point at with your wand.

The spell Dolohov used seems much different; while it was silenced, Hermione wasn't cut open by his spell at all, but was struck back... It seems more like a club than a knife (if that makes any sense).

Hope this helps.