For byte-sized elements, you can use sizeof(data).
More generally, sizeof(data)/sizeof(data[0]) will give the number of elements.
Since this issue came up in your last question, I'll clarify that this can't be used when you pass an array to a function as a parameter:
void f(byte arr[]) { //This always prints the size of a pointer, regardless of number of elements. cout << sizeof(arr); } void g() { byte data[] = {0xc7, 0x05, 0x04, 0x11 ,0x45, 0x00, 0x00, 0x00, 0x00, 0x00}; cout << sizeof(data); //prints 10 }