Hi gbbatoni,
Anything Gustavo tells you, take it to the bank 'cause he's one this board's gurus.
I'd like to add-on to his response.
I calculate the initial extent size for a table experiencing a high level of activity by multiplying # of pages used by page size, then multiply by 1.1 to immediately allow for 10% growth, and divide this value by 1000 to determine the initial extent size because extent sizing is in KB.
Using your numbers:
14,402,144 x 2k x 1.1 = 31,684,716,800
31,684,716,800 / 1000 = 31,684,716 is the initial extent size
31,684,716 / 10 = 3,168,471 is the next extent size
The 10% added immediately to allow for growth before you allocate another extent is arbitrary. You could use 5%, 25%, or skip it altogether.
This method has worked well for me, good luck
mike