grande (big)
maggiore (big)
massimo (biggest)

piccolo (small)
minore (smaller)
minimo (smallest)

There are other ways to express the absolute superior that dont require the -issimo form of the adjective. For example, the terms molto (a lot of), tanto (much), estremamente (extremely), or assai (very) can be used before the unchanged adjective. It depends on how dramatic the speaker wants to be! Compare: molto piccolo (very small) and piccolissimo (smallest)