Id go with the large fermenters.
at least double batch sized, this all depends on your brewhouse, Some breweries go up to 4 + batch sized tanks, as long as they are filled in a 24hour period the beer is fine.
the benifets of the larger tanks is:
- you can sqeeze more volume capacity than with more smaller tanks.
- Less yeast handling, less CIPing for the volume. Less work.
- loss is reduced. I have 14 -double batch tanks and 2 -4 batch tanks where I brew and I notice the volume better than 2 double tanks.
one thing you may consider is if your considering going with just 2 fermenters, you may have a hard time recycling the yeast since you wont be able to brew in time. I like the fact that I can pick from many tanks where I can harvest my yeast.
Also remember when planning your brewery that you should leave plenty of space to add more fermenters as sales increase. take into consideration how you will bring in the tanks too.