It's not the length of the poem that matters, rather it is the thought that goes into it. A short poem to your Valentine can easily be more powerful than a long one. It is the content that matters. In fact, if you're taking the time to write something of your own, you're probably on the right track. Don't go for the "roses are red, violets are blue" variety of poems. Don't give her a card with a prewritten poem in it. That won't get you anywhere. True romance are words from your heart to hers.