The "-issimus" (and related) suffix indicates the superlative form of the adjective. In English, the superlative is formed either by the suffix "-est" or the modifier "most".
Aren't there two forms? the "isimo" (grandísimo), which is generally just used to say "That's REALLY BIG!"; you're emphasizing how huge it is, and it's sorta more emotional, whereas the general "This is the biggest is just "lo más grande".
Generally speaking, I don't think it goes, "Este coche es grande", "Este coche es más grande", then "Este coche es grandísimo", I'm pretty sure you'd normally say "Este coche es lo más grande" for the superlative.
At least that's how I understand it in Spanish...