As a rule of thumb, in an open field, if the enemy has half as many troops, the advantage would warrant a D6 to the dice pool, while troops over twice the amount will definitely warrant 2D6
This seems a little bit ambiguous in English; do you mean 150% of the size and 200% of the size? Or just any significant advantage in troops giving at least 1 die and twice or more giving 2 dice?
Good job!