In the chamber of secrets harry watches phawkes (dumbledores pet pheonix) burst into flames and be reborn.......BUT later when he is IN the chamber phawkes is a fully grown bird how dus this work??

phoenix's r magical bird which die by bursting in2 flames and r then reborn from the ashes of its burnt remains.
phoenix's r birds with extreme strength and quickly growing ability

Harry sees Fawkes burst into flames near the beginning of the book and Fawkes rescues him toward the end - that's probably about 7-8 months!

well see it sometimes skips days if you remember in the Chamber of secrets movie it takes 1 month to make it yet it seemed only 2 or 3 days so maybe they did the same thing

Avian life cycles move much more quickly than a human's. Thus, it would make sense that a phoenix matures quickly. As this is not part of the phoenix's magical powers, it would not be listed in "Fantastic Beasts and Where to Find Them."
On the other hand, Fawkes may not have been fully mature by the time Harry wentf into the Chamber. Either way, you should just set this aside and not worry so much about it. After all, if Fawkes hadn't been able to get down to the Chamber withthe Sorting Hat, Harry would have died.

Their power is strong and the have the ability to grow and develop extremely quickly

A phoenix is a mythical creature. I say it can grow as fast as it damn well pleases.