If you argued that many programming languages already provide list.size() or equivalent (more so if I told you "in a language of your choice" before), I'd swallow my pride and accept it.
Then I'd suggest you try the other way, but it's a perfectly good answer.
The problem is, in Java and probably many others, that method computes the size in linear time by traversing the list. The javadocs for java.util.LinkedList note that this was a decision taken so that splitting a list at an arbitrary node, etc. could be a constant-time operation.
Then I'd suggest you try the other way, but it's a perfectly good answer.