Answer: The probability of having an offspring that is dwarf in size is zero.
Explanation: Since tall phenotype is dominant to dwarf phenotype, let T represent allele for tall and t represent allele for dwarf. A heterozygous pea plant will have Tt genotype while a homozygous tall pea plant will have a TT genotype. A cross between them will produce two homozygous tall pea plants (TT) and two heterozygous tall pea plant (Tt). They will not have any offspring having a dwarf size. Therefore, the probability of having a dwarf offspring is zero.
See the attached punnet square for more information