Frame size – how much really makes a difference?

I am looking around for a new bike because my current one is really too large for me, especially because I want aerobars. Which is too much of a stretch now: my saddle is currently all the way forward and I had to install a steep angle headset to get the bars closer to me to get comfortable. Also, seatpost is almost all the way down.

So this new bike I have in mind – the measurements compared to my current one are as follows:

The size “SM” is recommended for inseams 80-84 cm.
Top tube 54 cm, Down tube 53 cm.
My current bike has a 56 cm top tube and a 54.5 cm down tube.

This is only 1.5 and 2 cm difference, respectively.
My inseam is 82.5 cm (I’m 5’10’’ or 172 cm)
Will this really improve the fit for me?

Or should I try the S also?
The “S” size is recommended for inseams up to 79 cm
This has a 50 cm down tube and a 51 cm top tube.

This is a men’s bike. So how much does make a difference?

Ride lots of bikes. Then you can determine comfort and the differences between them.

I'm with Tulip-

I ride with a guy that's 5'4" and has, in his garage, a 48cm, a 52cm, and a 54cm in different brands of road bikes. All fit him fine.
There are also three different people on my old team who ride a size small cervelo TT bike- one is the guy I just mentioned, one is an "average" sized guy, and one is a 5'10" women!
My first bike was a 54cm Javelin that was almost too big, and now I ride a 54cm Blue that fits perfectly. The only difference in the geometry measurements between the two are a degree of difference in head tube angle and 1/2" difference in wheel base.

In other words, every manufacturer will be a little different! Go to a few different shops & try a lot of different ones out. If the shop isn't willing to let you take the one(s) you fall in love with out for an extended test ride of some sort, then look for the ones you love someplace else :)

I agree that you should ideally ride as many bikes as you can get your hands on. I would suggest that you look at some women-specific bikes, too. Are the bikes you're considering at a shop? If so, they should be willing to spend some time with you to get a good fit. If they're not, shop elsewhere. And unless you have a really good handle on what geometry works for you, I would encourage you to work with a shop or shops in finding the right bike and avoid buying online.

With your height, I'd be really surprised if you needed a "small," even in a men's bike. That said, men's geometry may not be best for you. Inseam only tells part of the story when it comes to fit. The length of your torso and arms, along with your flexibility, are important in determining a bike with the right "reach." Of course, there are other measurements on a bike that may make one more comfortable or handle better than another, too. Seat tube angle, wheel base, stem length, etc., etc. Some of that you can dial in, some of it demands that you get the right size bike to begin with.

But to answer your specific question, a couple of centimeters is actually a lot when it comes to bike fit and can make a tremendous difference.

When I bought my Specialized hybrid...it came down to chosing between a W's 53 and a M's 51. I wanted a step through. The clerk, at a very reputable LBS, told me I was fine on either.

HOnestly, I felt a little better on the smaller bike but was worried about the top tube so I got the W's....and I think it's just a little bigger than ideal...

I discovered this when I bought my Jamis...a 47...and I am just SO much more comfortable on it.

a couple of centimeters in the wrong place can make a BIG difference.

Well, I ended up getting the "S" because it was significantly shorter top-tube wise than my old bike.

I also just got back from cycling hols where I rented a 52 cm, not a 54, and it was almost perfect, although fitted with a rather long headset.

So my bottom line is to go "smaller" if stuck in the middle, you can always install "larger" parts.