How long was a cubit?

How long was a cubit? I've seen scholars use conflicting criteria. On the one hand, they generally define a cubit as the distance between the elbow and the fingertip. On the other hand, they define a cubit as about 17.5 inches. 

Problem is, if you define a cubit as the span between the elbow and the fingertip, then that would be relative rather than absolute, since it would vary depending on the height of the individual. As a rule, taller people have longer arms and longer fingers. In addition, people in the ancient world were generally shorter than their modern counterparts due to poor diet. And I've read that ancient Jews were generally shorter than some other people-groups in the Mideast. 

This can be relevant to questions concerning the size of the ark or the height of Goliath. Unbelievers object that the size of the ark would exceed the structural integrity of wooden ships. (Mind you, the text doesn't actually say the ark was made of wood. The Hebrew word is a hapax legomenon.) If, however, the cubit was shorter, then the ark was smaller.