I recently bought an older road bike ('88 Miyata 712) with very low miles and great condition. Frame size is 56 cm center to top.
I've been riding it now for approximately 150 miles.

Today I learned that I had took my body measurements wrong prior to buying this bike.
I am 5"11 and my inner leg is 2.82 feet ( 181 cm and 86 cm respectively). My long legs in relation to my height seems to be the problem.
My arms are also longer than average for my height.

According to most web resources, I seem to have bought a too small framed bike for my use. A suggested size (classic road bike frame style) is usually a 58 or a 59 cm.

I got the seat height right for me by fitting a longer seat post, but I'm wondering if this bike is going to cause me trouble as it is.
The bike sure looks funny with 15cm (almost 6 inches) of seatpost showing.

Do you think the size will be a problem for me on the long run? My goal is to ride at least a hundred miles a week.

As it turns out, by using the web calculators, the bike actually looks almost ideal size for my wife...
But If I end up hunting for a new, bigger frame for myself, how do I know I get the right size? I will be shopping in the used market for sure.