Strawberries are best fertilized after harvesting as too much nitrogen in the early spring can cause problems with the fruit being soft. Using 12-12-12 is fine.
Strawberries do want moisture - not deluges - just a nice even, consistent moisture.
Your description of the insect is a bit, uhm, vague. I'm going to take a wild guess and say that maybe you are dealing with pill bugs aka roly-polies. These are the taupe or gray coloured insects that are often said to look like miniature armadillos and tend to roll up when disturbed. They are chewing insects and are known to damage soft fruit and are attracted by moist soil and debris and ripening fruit.
I would discourage the use of Sevin and would try clearing the bed of debris; leaf litter, etc. in which the bugs live and apply a dry mulch such as straw or hay or pine needles to previously well-watered soil. If the infestation is vast I might try a bit of diatomaceous earth judiciously sprinkled beneath -not on - your plants.
Berry size is not just a matter of fertilizer; but also plant genetics. So no matter what or how much you feed a small-fruited variety you 'aint gonna make it big and some of the best tasting varieties produce small to medium size fruit. That said, and in the absence of knowing what variety you are growing, you could probably 'maximize' the size of your berries by thinning clusters of fruit so the plant then directs its energy to fewer, but larger, berries. That would be a major pain in the -whatever- and I don't know anyone who does this.
Hope you have tons of fruit!
