plug in the geo and your stem dimensions for each of those sizes at
Stack and reach calculator play around with spacers and stem angles until you get something that is close enough. the results are theoretical, so don't expect this to be 100% accurate, but it should give you a good idea of whether it will work or not.
without knowing the specific model and year of your Venge, I can't be certain. in general, when comparing two bikes of the same model:
- a smaller frame will have move exposed seatpost, which can be a good thing because the seatpost will flex and give more with the terrain
- the smaller frame will take a little more spacers/ stem height to get the handlebar high enough due to the typical shorter head tube (shorter frame stack). it sounds like you have the stem pretty low already, so getting a smaller frame probably wont' require and excessive spacer stack or taller stem
- you'll have a little more standover height. not a big deal.
- the smaller frame might fit with a more average stem.
I went through this recently when I bought my current drop-bar bike. I like a fairly compact fit, so I bought a 53cm Twin Six Standard Rando. I have been experimenting with 80 and 90 mm stems on it. 100mm is just a bit to stretched out on this bike. for my height—5,9", same as you—I could have fit on a 55cm frame as well, but with my tight hamstrings, it would have required a stem that is shorter than what I think is appropriate to optimize handling on such a bike. my bike has little in common with your Venge other than the drop handlebar, but I use this example of how two sizes could fit the same rider depending on the type of fit you want.
stem length is a contentious topic and I don't have the answers. some people say you can put as short or long a stem as you want on any bike so long as it makes the bike fit you. sticking with
conventional wisdom, I think that the stem should be proportional to the rider and for an average height person like you and me, 90–110mm is a good range. if you need something much shorter or longer than that, barring a physical deformity or other limiting factor in your body, the frame is likely the wrong size.