Hello. I have what I believe is a 1991 Cannondale ST600. Can I have some help decoding this serial number. In particular, I would like to know the year and frame size. It says:

B1-0302
02105911740?

Thank you.

Can you post a picture of the entire bike taken from the drive side?

Yes. It is partly disassembled at the moment.

the vintage cannondale site has a page which explains how to decipher serial numbers:
https://vintagecannondale.com/info/serial_numbers/

yours falls under "version 2"
02105911740
0 = 130 rear spacing
21 = frame size
0591 = date of manufacture May 1991
1740 = sequence number

https://vintagecannondale.com/year/1991/1991.pdf

Thank you. I wonder how they figured it is a 21 inch size. I measure 20.25 center to center and 22.25 center to top.

I've read that Cannondale sized from the top of the BB to the top of the seat tube.
So it was seat tube length
C-T is usually to the top of the top tube join on the seat tube.
C-C would have made the frame a little bigger with the large diameter tube, and the long seat tube neck above the top tube could have thrown people off even worse.
Seems to work, and explained to me why they always seemed to be a little smaller than expected.

1991 catalog shows right color (light metallic blue), but sloping crown fork. 1992 shows correct unicrown fork, but only comes in "Champagne". Looks like they switched this frame to a unicrown fork part way through the 1991 model year?

My 1990 ST600 has the original unicrown fork and looks basically the same as the OP's. I'm not sure exactly when Cannondale started putting unicrown forks on the touring bikes but it was sometime in the 80's and I don't think they changed much.

The OP's can't be a 1992 as the Cannondale decal moved from the top tube to the downtube starting in 1992. Cannondale, that same year, also changed the name of the touring bikes slightly to just have T instead of ST.
1990 had the ST600 decal on the chain stay while in 1991 it moved to the top tube.
Another distinction in 1991, for the touring bikes, is that was the first year for 700c. My 1990 came with 27 inch wheels.

Oh and a note on sizing, Chuckk is right, Cannondale's officially listed sizing (at least for their 80's and 90's stuff) is measured from the center of the bottom bracket to the top of the top tube as measured along the length of the seat tube. I've got my ST600 with me at work right now and I double checked the measurement and it's a spot on match to the listed size of 25 inches.