If this is the "classic" power rangers (before Zordon was killed by the Red Space ranger... on Zordon's orders!), the Rangers could be teleported through time from the Command Center to stop some sort of monster... which will then have to grow... this fits well because the first season of the power rangers used the dinozords, including a dragonzord (an an ultrazord)... but if we will keep it limited to five, we ought to just have the five main dinozords (only two of which were representative of dinosaurs!) combine to form the megazord.
For those who don't know, the five zords were the Tyrannosaurus, Triceratops, Pterodactyl, Mastodon (Mammoth in the original Japanese sentai), and Saber-Tooth Tiger. There was also a dragonzord (think mechagodzilla) commanded by the green ranger, who could summon it by using a musical dagger... and a sauropod-like zord (with wheels) called Titanus which could combine with the megazord and dragonzord to form the ultrazord. That's seven zords in total...
EDIT: Of the original rangers, There were red (Jason), black (Zack,who was African-American) , blue (Billy), yellow(Trini; she was Asian), and pink (Kimberly; yes, a girl pink ranger; they've had at least 3 pink females in the land before time series as well.) At first, it was three guys and two girls (sort of like the original Gang of Five.) Later, the green ranger joined, and he increased their numbers to six.