One of my favorite florists,
Zezé, 938 1st [52nd] 212.753.0067 not only handles flower arrangements for weddings but also offers a small but magical carriage house as a private dining room. How romantic and cozy that would be for a small wedding reception! Its can hold up to 40 people for a sit down dinner or up to 100 for a cocktail party. You can see the space through one of their approved caterers sites:
http://www.bellinyc.com