I have been using an older 5' model and got along great. Recently switched to a track tractor and 6' model. I haven't used it yet though.
My 5' model did everything I needed but it was hard to go much over 4-4.5' or else the dirt boiling up would start to interfere with the frame. That is why I went to a larger model, for a little extra room around things at my normal depths. Actually I don't think I could even get a smaller one setup for a track tractor.