I am baking a birthday cake that needs to feed approximately 45 people.

What size rectangular pan would YOU use? It will be a one layer cake...

I think it depends on how big you want the pieces to be. When I've done 1/2 sheet cakes (12x18 pan) for weddings, I score them to be cut into 48 pieces.

Hope that helps!

45 servings from one layer? I have a 11"x15" pan, and would bake two of those (for a 15"x22" cake)

This would make 55 3x3 one-layer servings.

Now that I've gone back and read your original post, I think I would agree with edencakes.

Typically the pieces I serve for weddings are around 2x2, because it's served along with a full meal. If you need bigger pieces, I'd go with two of the 11x15's.

Sammy