Re: size of wakeboard for a 14 year old
It's really not that specific - plus - width can be a factor too.
Just don't get anything huge (143cm+) or tiny (<134cm) and you should be fine, especially if you're just learning.
My 6 yr old 40 lb daughter got up on my 143 w/o any trouble. My 145lb wife rides it as well.
At 190-200lb I've ridden a 137, 141, 143 and 138. There are certainly differences in ride based on design, but nothing I could attribute specifically to length.