Two words that pepper our day-to-day conversation are the linking verbs "is" and "are." (Note that "is" ranks 7 and "are" ranks 15 in the list of the 100 most frequently used words. "The" ranks first, followed by "of," "and," and "a.")
Given the importance of these words to our lexicon, I'm constantly amazed at the pushback I get from clients who apparently were taught in elementary school that these words are too small to deserve capitalizing in headings, subheadings, and titles (when not in sentence case). It's time to let it go.
The length rule applies only to less important words, such as conjunctions and prepositions. Moreover, if you're going to use length as the basis of your argument, then we'd never capitalize verbs like "run" and "eat."
I'm not standing alone on my soapbox: for example, the Chicago Manual and AP Stylebook agree.
So, the next time you're typing a title or heading, please think twice before discriminating against the poor, defenseless (yet critically important) linking verbs.