My Bottechia has a 58 cm frame, and the top of the seat is 6.5 inches above the top of the seat tube when I ride (I'm 5'11"). I saw a desirable craigslist bike with a 53 cm frame. That is definitely too small, right?

Yes, it sounds like it is.

I'm 5'11" too and I've been successfully on a 54, 55, 56 & 57. I 53 might be a bit too small. Did you get specific measurements of the top tube?

I'm 5'7" and ride a 53cm frame.

You might could ride a 56 but not a 53......keep looking-you'll find the perfect one. Just be patient.

Way too small.

Bike sounds perfect, but you're too tall.

Yyou may be ok! how long is your torso? I know lotsa guys your size that ride a 58 and some that ride a 59 or 60 and some that 56 or 54 . I'm 5'8'' was 5'9'' till my knees went south I've ridden 54cm to 58cm frames, admittedly the 58 sometimes made my voice a half note high sometimes. I recently had a custom seven made for me it has a 47 cm sloping seat tube, (probably about 52 cm effective) and a 56.5 cm top tube, which is way off "square" (normal), and I run a 13 cm stem. Greg Lemond is my height and rosw a (53 ctr to ctr/ 54.5 ctr to top) in his glory days. Most bikes have top and seatubes in lengths that are within 1cm of each other. Many/most of the reputable "experts" think your torso measurement in more important than the length of your leg, after over 35 yrs of riding, touring and racing I tend to agree. If you like how the bike rides and have room in the "cockpit" ( Can you stretch out enough?) Don't worry about it!!!! Lots of pro have a lot more seat post "sticking out" than that. If you like the 58 the 54 may not work, ride it see how it feels!

everyone is different

53 is too small for me and I am 5' 11 3/4" barefoot

Inseam?

30" inseam and I ride a 51 conventional. But if this is a Compact frame it can fit larger. I ride a Small Compact Giant that goes up to 54 conventional.

31.5"

I think I'll take a test ride, mostly to have something to compare other bikes to. I'll leave my money at home to avoid rash decisions.

When he puts his seatpost at its max height, the distance from the saddle to the pedal (at lowest) is one inch greater than my current seat measurements. However, I realize that there's more to it than that.

Just remember inseam figures used for bicycles is bike inseam. My bike inseam is about 1 1/2 inches longer than my pant inseam.

I have found this chart to be fairly accurate on sizing vintage bikes.

It's probably too small but if the 53 isn't the same brand as your 58 you cannot be sure without measurements.

Seem to be posting these pics a lot at present but they do show the extremes of my bikes. Boreas is a 51 cm with a 53.5 cm top tube and has a 90 mm stem. It fits me perfectly and same with the TCR except that has a 100mm stem. The OCR is an extra small Giant compact frame and covers from 46 to 51on conventional frame sizing. Don't ask me why Compact frames fit such a size gap but they do. The OCR is just a fraction small on top tube length but instead of putting a longer stem on- I raised it and it is a comfort bike for short local trips. So with your extra 1 1/2 inseam to me you may be ok- but test before buying and be prepared to fit a longer seat post and possible spend some time sorting the bar stem to length and rise.

Well, it turned out that it's a 54 cm frame, and I bought it for $375.

Last night I measured all the distances on my bike (seatpost to handlebar, etc.), and with the seatpost up high on this one, all the measurements were very close. I also took it for a nice long ride, and it felt good. Only time will tell if I was crazy, but you should know that there are very few road bikes on craigslist out in the boonies here, and I've been watching for one like this for about a year.

This is what it looks like with no clean up or changes: