Well, n-heptane boils at 98°C but you'd have to look at the label on your naphtha/pet ether to compare. As a mixed hydrocarbon solvent it will have a boiling range according with its composition.

Allowing your recrystallisation to cool (yet) more slowly will result in larger crystals forming. This would include using pre-warmed petri dishes, and insulation to slow the cooling. I see that you are otherwise on point with the slow cooling method.

You could even save a couple of your current batch to act as seed crystals.