EltonJ's answer is correct for past editions of RM, but the question is clearly about RMU, for which that answer is false.
Minuscule is the minimum attack size. A Shock Bolt against a dragon? Let's assume a fairly small dragon, of only Big size. The size adjustments for the Shock Bolt are already built into the Minuscule Lightning Bolt table and we don't have to worry about them. If the bolt hits, the dragon suffers the number of hits listed and takes a critical reduced according to its size. As a Big creature, this dragon only has the critical reduced one step. If it were a Huge dragon, the critical would be reduced three steps. (See Table 9-4b.)
Iuzbul, you are correct about Y criticals. I pointed out in the Core Law release type thread that these were not explained in text, but after the "final" version was completed. This may be the most important erratum.