I'm 5'10" 190# and I have legs that are a bit short compared to my torso. I have a 4" and a 3" from best belts. I bought the 3" for dead lifting after several workouts in a row where I felt like I couldn't get a good set up without seriously hurting my ribs using the 4".
The 3" defiantly makes it easier to get into the correct DL set-up position but the belt itself is very uncomfortable. I think the edges of the belt dig in much more because of the narrowness. I can also tell a difference in level of valsalva support. I've gone back to using the 4" exclusively. I just wear it a hole looser and position it higher in the back and loser in the front. Some times it takes a few tries to get it in the right place but it's worth it.
All that being said... this will be highly individual. Best case scenario would be convincing a training partner to buy one so you can try it first.