Actually if you know that much (seeing posts waaaay up there) your sizeof() attempt almost works; the basic tactic for seeing the number of bytes consumed by an array is something like so:
If *all* you have is a ptr then you are screwed unless you get fancy and have debugging turned on, find the bounding array flags (how the debugger knows when you have overwritten the ends of an array), dereference the original pointer and start walking memory till you hit the marker.
double array; // for example
int size = sizeof(array) * 10;
The only thing more fun than debugging with the proper tools is debugging without them.