If they aren't the same size, then you won't be able to match up the entries.
Check it out:
You can't do this! Each guy MUST have a partner!
Hmm... So, does this mean that we can add a 1 x 2 matrix and a 2 x 1 matrix?
|Can we do it?|
No! They are NOT the same size! (And, no, you cannot just tip the second one on it's side, so stop thinking about it.)
Add or subtract these,