overlength brass can extend beyond the chamer, into the barrel, where there is not enough room for the brass to expand and release the bullet. This means higher pressures, and can be dangerous.
A max length case should chamber and fire ok, BUT since there are tolerance differences between chambers, we trim the brass to ensure safety in all chambers.
New brass nearly always stretches (and stretches the most) on its first firing. It may not strech again for seveal firings, or it may stretch a bit each time. Your loads, and the fit of your chamber and sizing die are the major factors in this.
All else being equal (and it almost never is) bigger bullets tend to work better.