One More Measurement Of Interest...
While I acknowledge DX-MAN's timely response and helpful attitude, I must disagree insofar as he ignores what many consider the more important measurement in fitting a bike.
And that is the [effective] length of the top tube plus the reach of the stem, which give the proper handlebar relationship to the saddle. The right starting point for this is a bit more difficult to discover for a new rider, because you can't simply measure from a bike that you know fits you correctly. Have your bike shop explain the top tube length and stem reach for a bike that feels like a good fit for you (after having ridden it outside and on a trainer under a fitter's supervision). That measurement must factor into your purchase decision.
For me, at 6' 3/4" tall, with a slightly longer torso, the proper combination of top tube + reach = approximately 68.5 cm. That's for a horizontal top tube, measured center to center, and a handlebar height that's only a few (4-5) centimeters below saddle level. So a 58cm frame size, with a 57 cm top tube, wouild require a 110mm stem, approximately.
The standover and seatheight measurements matter, but not so critically, as there's more room for adjustment. Some can ride a bike with little to no standover or a very long seat post showing. But with the wrong forward reach...it won't fit.
Not knowing the geometry of the bike in question, I'd stick with the 55 or 56 until you discover what feels correct to you. Then take your correct measurements from there.
My two cents. Good luck.