Sorry I was a bit rushed lastnight when I responded. I have made posters that size before (only a couple) but with very good results using the OnOne Perfect Resize product. The product is not overly expensive and gives you tremendous freedom in printing and sizing options (including auto borders for canvas wraps).
Quoting the OnOne website
Enlarging Images
Perfect Resize 7 - The next generation of Genuine Fractals is still the industry standard for image enlargement. Resize your images 200%, 400%, 800% or more without the loss of quality you get with other resizing methods. Perfect Resize works with Photoshop, Lightroom, and Aperture or as a standalone application.
I have used this product and it works well, their website is
Perfect Resize 7 - onOne Software and they do have free trials.
Best of luck