zen_

I'm 5'9" too and would never buy a large; learned the hard way with an internet buy a few years ago that even a "medium large" was too large. Like other have said though, it's going to vary a lot based on the actual geometry of the bike and you preferred riding position / flexibility. You can make a bike that is a bit too small bigger with adjustments to the stem position, but you can only make a bike that is too big a little smaller before bad compromises have to be made.

Hiro11

I'd ignore the sizing label, determine the stack and reach numbers for a bike you own and like and then buy the bike that gets you in that ballpark. To answer the question: I generally size down if I'm exactly in the middle between two sizes. This is not advice, I'm just explaining what I do.

urbanknight

If you insist on buying without testing it first, read geometry charts and learn where your proportions fit with respect to top tube length, reach, seat tube angle, etc. Not only do different manufacturers have different ideas of what makes small, medium, and large, but also proportions within those as well.

Calsun

It is a subjective decision based in large part on how you like to ride. When I started doing long distance tours in the 1960's the bike frames had criterium race geometry that was very tight and not great for long rides. I would go with a larger frame to be able to be able to stretch out when riding so my lungs and stomach muscles could work better. I always put new handlebars and a new stem on my bikes but wanted a "reach" that put me in the ballpark. I did not want to be hunched over the handlebars. The Dave Scott Centurion was the first factory bike that felt great from the start and before making any modifications having been designed for use in triathalons. The frame geometry that would be my first choice at 5'9" is the Trek Roubaix in size 54 for long distance rides.

50PlusCycling

I am 5,9' and using the old Fit Kit formula it recommends a 56cm frame. This formula varies depending on things like leg and torso length, so it can recommend a longer or shorter seat or top tube depending on your body type. But generally speaking, having the same size seat and top tubes will put you in the ball park. I have ridden smaller frames, but I don't like feeling scrunched up on the bike, and don't like using long stems to compensate. When I ride larger frames I don't have enough standover room over the top tube (not relevant with sloping frames), but I like the more stretched out position. A good bike shop should be able to measure you and recommend the best frame size for your body type, it is wise to get the best fit you can.

Calsun

I would not trust a bike shop worker to properly fit you to the frame as they are subjective based on their own experiences and personal preferences. For me the two key variables are the head tube angle and rake and the length of the chainstays with shorter ones better for hill climbing. Standover height is a nice to have but I do not stand with both feet planted on the ground over the top tube of my road bikes.

At least if you buy a used road bike you will not lose any money if months later you decide to sell it for any reason.

GhenghisKahn

One can do the opposite as well. Found a suitable frame height for my partner, but the 'cockpit' was all wrong for her proportions. So, we replaced the 120mm flat stem w/a 70mm riser, saddle w/a WSD, lowered the seatpost and moved the rails as far forward as possible, cranks from 170 to 165mm and the 12-25t cass w/a 12-32. Made all the difference in the world to her. She says the bike fits her perfectly now.

But, the best option is to get professionally fitted by a lbs that has the equipment. Doing utility or even reasonable commutes don't require all that much fitting. But, if spending long hours in the saddle on or off road, touring, bike packing an ill-fitted bike can ruin an otherwise pleasant experience.

BCSC

I'm 5'10" and have long ridden a 56 with no problems. But I'm in the market for a new bike and based on my research and several test rides, a 54 (at least with most makes) is a more "ideal" fit for me.

PeteHski

That may well be the case depending on what your idea of an "ideal" fit is. I'm guessing you simply prefer a lower stack height and shorter wheelbase. Because that's what it comes down to when you can fit on more than 1 frame size using different stems and spacers.
I'm just a fraction over 6' and can comfortably fit on a 56 or 58 in most road frames. But my "ideal" fit is usually on a 58 simply because I'm going for an endurance fit with slightly higher stack and longer wheelbase. There is no right or wrong answer to sizing up or down. Only personal preference and riding goals.

410001661JW

I just bought a 2021 Giant Contend and I was looking for the M/L frame and it was not until when I went in to actually buy my bike that the guy did a quick fitting on my and said you are better with a Medium and it did fit way better. The last bike I bought was a 1988 custom made Cyclops so I had nothing to go by.

Rdmonster69

I have a 58 older Trek and a newer Trek 58 Domane. The new 58 feels way better than the old 58 most likely because of the stack/geo differences. I am almost done with a 60 CM Litespeed build as I always felt a 58 in traditional geo was a bit cramped feeling.