I've picked up a road frame with an interesting (to me) geometry. I'm trying to decide whether to build it up for myself. I mocked up the dimensions in AutoCAD so you can see. I'm a little over 5'9" with a 32.5" cycling inseam. Is a 51cm top tube too small for me, or can I make it work? I used 170mm cranks and a 120mm stem with 17 degree rise in the drawing. The stem height is 3cm, and the seat to handlebar drop is also 3cm.

I currently ride a 56cm with 90mm stem, that fits me reasonably. The handlebars and seat are the same height, and I ride primarily in the drops with no reach issues.

Thanks!

Most conventional recommendations would have you on a bigger frame than that. The seat tube looks steeper than on larger bikes, so you might need to set back the saddle a fair ways, depending on what works for you.

I suggest you take the CAD system and replicate the distances (vertical and horizontal) on your 56cm bike from the bottom bracket to some point on the saddle and to the handlebar tube centre. If you can replicate these on the new frame, what length of stem and how much saddle setback will you need. If you need unusually large saddle setback and a really long stem, that might be a warning sign.

Good idea, I'll try to overlay those dimensions and see where I end up.

assume you are talking Road bikes .. Once I figured out what was right for me I dont own anything else ..

For my leg length, 56 cm frames are best. For my torso length, 58 cm frames are typically best. I ride bikes from 52 mm to 59.5 mm. The 52 cm is too small but with saddle all the way back and a long stem, it is okay for shorter rides, typically my rides on that bike are 25-50 miles, and it feels racy and quick handling. The 58 cm and 59.5 cm frames are comfortable for any distance ride. I think most of us can more or less comfortably ride a pretty wide range of frame sizes, as long as the saddle to pedal distance is right. But I wouldn't take the 52 cm on a century.

May I ask your height and inseam?

5' 11"
Bike inseam in bare feet 33"
Preferred distance from top of pedal surface to top of saddle surface at the center of the saddle is 34.5"
On Brooks saddles, a bit higher because they sag a bit
On bikes I ride in cycling shorts, a bit lower because of the pad's thickness
I use SPD pedals on most bikes, and bike shoes with recessed SPD cleats. If riding with casual shoes, like Vans, then the saddle feels too high.
I have large feet (size 45 euro/10.5 US) and tend to pedal toe-down