It helps regardless of forum size, but be sure it is somebody who you know and trust.

Not only will a partner help you with the work aspect, but a partner is a guaranteed active poster...thus helping your posting activity.

That said, I chose to go without a partner for the first 2 weeks of opening my most recent forums...I did this because I didnt have anybody that I knew well enough to bring on board. Just recently, after establishing the forums, I decided to "hire" somebody and it has helped wonders.