Hello,

Well, take a look at the sizing chart of the frame on page 15 of this PDF: http://vintagecannondale.com/year/1989/1989.pdf

Depending on whether the guy gave you the correct measurements, it seems like the frame is at least a size 60 (based on the 33" height from floor to top tube), potentially even a 66, if the measurement of the top tube of 24" is correct. That is a VERY very large bike frame, the largest sizes they make for that frame. I am 5'6" myself, and I have the size 54 of this bike (which is about a small to medium size), and that is even on the big side for me.

Do yourself a favor, and get another bike, because this one will be way too large for you, and will be very, very uncomfortable to ride. Given your height, you need a frame of aproximately 54 cm seat tube lengths (measured from the axle of the crankset to the top of the seat tube).

If you really like this bike, try looking on Ebay for the same bike (Cannondale 3.0) in size 52 or 54.

Good luck!