solitaire-

Plants are like reptiles, the more space you give them, the bigger they will grow - Now, unlike reptiles, the longer you veg them, the bigger they will grow in flower. In your case, 55x30 is 16 sq ft, or about 3 square meters. If your doing a large sativa to full growth, you can do 3 nicely sized plants

However, if your doing a scrog, you can fit alot more - Or if you plan on keeping them between 3-5' you can do alot more. It all depends on your grow style. I've been doing a perpetual SOG in a 18"x42" cabinet (36" tall) and I can fit 12 plants in it, and I can harvest every month. BUT - I can fill that space with 1 plant, that will give me 3-4x more bud, and harvest every 2 months

So really, it depends on how you want to grow. I prefer multiple strains and flavors and like the constant supply