If ‘B’ gene is incompletely dominant for starch grain size then ‘Bb’ genotype represents which kind of starch grain

Pleiotrophy: a single gene product may produce more than one effect.
Starch synthesis in pea seeds is controlled by one gene with two alleles, B and b.

  BB - effective synthesis of starch - large starch grains.
  bb - small starch grains - ineffective starch synthesis.

Heterozygotes Bb, the starch grains produced, are of intermediate size in Bb seeds. 

So, if starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance. 

Bb – intermediate size starch grain.

The same allele also controls the shape of the seed.

BB/Bb shows round seed at maturity 

Thus for shape of the seed alleles B and b shows dominant and recessive relationship.