You can try doing a fitting on yourself by using an online fit calculator:

http://www.competitivecyclist.com/za...LCULATOR_INTRO

Ultimately, riding the bike will tell you what it feels like. Manufacturers use different ways of measuring what a 54 or a 56 is. Some will use bottom bracket to top tube while others will use top tube length. So a 56 by one manufacturer could be a 54 by another depending on which measurement they are using. I would look at these two components, top tube length and bottom bracket to top tube length. This will tell you how much you need to raise your staddle (bottom bracket to top tube) and how long of a stem you may need (top tube length).

It also gets more complicated on how these things are measure like center to center or center to top. And with sloping top tubes the mesaurement is the "virtual" length. They usually have these measurements on the frame manufacturer's web site.