Multidimetional arrays : .length

How would I go about checking the length of a sub array? If I have a code like this:
ActionScript Code:
[FONT=Courier New][LEFT]a = [COLOR=#000000][[/COLOR][COLOR=#000000][[/COLOR][COLOR=#ff0000]“a0”[/COLOR],[COLOR=#ff0000]“a1”[/COLOR], [COLOR=#ff0000]“a2”[/COLOR], [COLOR=#ff0000]“a3”[/COLOR],[COLOR=#ff0000]“a4”[/COLOR],[COLOR=#ff0000]“a5”[/COLOR][COLOR=#000000]][/COLOR],
[COLOR=#000000][[/COLOR][COLOR=#ff0000]“a0”[/COLOR],[COLOR=#ff0000]“a1”[/COLOR][COLOR=#000000]][/COLOR][COLOR=#000000]][/COLOR];
[/LEFT]
[/FONT]

and I wanted to check the length of the second subarray (the answer would be 2) what would the syntax be?

Edit: the title should obviously be Multidimentional.