Originally Posted by Andy_K

Yeah, that's exactly what I meant.

Looking at your bike you'd really need to go down to a 48 to get a typical amount of seat post exposed (the 51 only gets you another inch), and in the process you'd get lower handlebars, a steeper seat tube angle and a slacker head tube angle -- none of which are likely to be positive.

That's not to say the 48 wouldn't fit you better by some arbitrary standard, but there are positives and negatives. Since you already have the bike, you're better off keeping it unless there's something that feels like a problem to you. Things that just look like problems in a picture are much less important.