In the case of a short machine language program, you just count the bytes to determine the length of code. This method is not practical for longer programs, though; for such programs, hexadecimal arithmetic is more useful. If you're familiar with hex digits (A = 10, B = 11, C = 12, D = 13, E = 14, F = 15), you can subtract hex locations, just like decimal numbers.
To discover the length of a longer machine language program, simply add 1 to the last location used by the machine language program, and then subtract from that the first location used by the program. For example, an ending address of $0357 and a starting address of $0300 would yield the calculation ($0357 + $0001) - $0300, or a length of $58:
last + 1 0 3 5 8
first - 0 3 0 0
length 0 0 5 8
Likewise, the length of a program with an ending address of $03AC and a starting address of $031B is $92:
last + 1 0 3 A D
first - 0 3 1 B
length 0 0 9 2
In each case above, the answer is the length of the machine language program, and you may use it in a BSAVE command:
BSAVE PROGRAM, A$0300, L$58
You'll find more extensive explanations of hexadecimal arithmetic in most introductory machine language books. Check with a local book store or Apple dealer to help you locate one.