Road Cycling - Bike size

I am an avid MTB's who recently purchased a used road bike off ebay. In a quest to get my money's worth I bought a bike I new was too small. I installed a longer seat post and wider handlebar. I have just started riding it this week. I've been on two 20mile rides. The bike feels good. The more compact geometry is nice and I have had no discomfort. I was sized for a 60cm and bought a 56cm. At what point is the size going to become a problem if ever?

As you can adjust the seatpost up and down, height is not such a critical dimension, but the length of the bike (top tube + stem) is harder to adjust. You can replace the stem within limits (usually +- 2cm).
Usually shorter bikes have a shorter top tube, but the 2 figues do not maintain the same proportions across the whole size range. Check out any bike geometry table from a manufacturers website (eg Trek) to see.

Small bikes also have shorter head tubes, so you can set the bars lower, if you wish to ride in an extreme aerodynamic position.

Judging from pictures a lot of pros do the same. When you see a bike with the
saddle 5" above the bar and sticking out 6" above the top tube the bike is
'undersize'. Use of aerobars also mitigates the shorter top tubes. Steve

Dont confuse undersized with compact. Compact frames are designed to be low, but not short.

Just make sure you have the proper reach.. but reach is very a personal matter..

But I guess using a stem 2-3cm longer will fix it..

Pros usually have custom fit bikes.. Even though some are production models, those bikes fit their bodies..

They need the seat to be higher than the handlebar to attain a more aero position.. But I don't think they purposely pick smaller frames..