It cannot be unlocked.
Under the FIA regulations for Brake discs:
11.3.2 All discs must have a maximum thickness of 28mm and a maximum outside diameter of 278mm.
Your brakes are already as large as it can be without breaking the current rules of the sport.
That option in the game's menu is probably redundant code, from a time when brake sizes could be changed.
All the real F1 cars have a brake size of 278mm already, to put bigger brakes in would break the rules (pun intended :tongue
.
So the game correctly reflects real world rules of the sport.
If it is unlockable as James suggests, then it shouldn't be, in my opinion if EA are going for authenticity.
<edit> In fact 2007 was the last time that the brakes could be between 300mm and 305mm. In 2008, brakes had to be a max of 278mm.
The new 2008 rules were announced mid 2007. So coding for the F1 game could have started way back in 2007, which had brake size changes. If the rules ever change again, the code is already in there.