There'll be others here who know this stuff much better than me, but having worked my way through the same kind of issues from moving from old MTBs to modern/modernish road bikes in the last few years, my thoughts would be:
Manufacturers nowadays use the numbers they use to designate frame sizes (55, 58 etc) to refer different things, and they won't tell you what - you have to determine that from the geometry chart so finding that is crucial. Traditionally the frame size number would refer to the seat tube length, but with the variation in modern road frame designs the top tube length is sometimes taken to be the single most significant dimension, in general. So some manufacturers might still use the size number to refer to the actual seat tube measurement, but others to top tube measurement - and usually the 'effective top tube', since many frame designs now have sloping top tubes. More confusingly still is when the manufacturer's frame size number refers to neither of these, but to the corresponding bike size in 'old money' (seat tube): so if you've always ridden old 58cm frames, say, then you'd still be suited to that manufacturer's size 58, even though there might not be any relevant dimension on the frame that is actually 58cm!
What it boils down to is that the sizing and geometry of each frame is very specific to that model. The only way to be fairly confident about what size is best is to look at the manufacturer's geometry chart for the specific model you're interested in. That said, my own experience is that modern road bike sizing is pretty forgiving if you're willing to switch seatposts (setback/inline) and stems, and then if you buy a frame with an uncut fork or one with an inch or two of stem spacers already present (failing that replace with a stem with a different angle, or even flip it) then you should have enough scope for adjustment along all the main dimensions to make it fit well. In any case, everyone likes their bike to fit a slightly different way so to a certain extent you need to do that tweaking anyway. For many manufacturers there's an overlap where two sizes of frame can be made to work well: I know I can fit both medium and large Canyon road frames well for example. I'm a fraction under 6'0 and over the past few years I've made frames with top tubes between 53.5cm and 56.8cm work absolutely happily as far as comfort and handling are concerned. It doesn't sound much but it's quite a big range in terms of sizing - a smallish medium to a fairly large 'large'!
Though if you're looking for an exact/professional-type fit with a specific length of stem to produce a particular feel to the steering etc, then that would be a different matter of course. I'm not enough of a purist for that and am happy to buy close enough to the right size and tweak it until perfect!