As pointed out in the European proposal, the larger the antenna the longer the baselines in the smallest array. This means that, for example, an array of 15m's is only sensitive to subarcsecond structure in the submillimeter. We want to minimize this problem by having dishes as small as possible. Even so, mosaicking and total power observations will be important for many of the observations we are interested in. A larger antenna would produce a smaller number of antennas than we would be happy with and an array of antennas too big for much of our science.
Anomalous refraction becomes more of a problem for pointing as one goes to bigger antenna sizes.
Even at 10m, these errors will make it difficult to meet our pointing spec.