I'm going to be purchasing a cannondale slice through a friend that is a cannondale dealer, he is the owner of a ski and bike shop with more of an emphasis on ski so he doesn't know too much. I'm going to get the bike adjusted professionally to me once I have it, but need to know what size frame I should buy. From what I've found on the Internet, I should
Be a 58. Any suggestions, I am about 5-11/6 ft

Welcome to the forums!

Getting fit on a road bike requires more than just height. Tri/TT bike more so. Torso length, upper/lower arm length, and flexibility figure into your Tri/TT bike fit.

Not knowing anything but your height I would think 58 is too large for you, a 56 would be better IMO. A smaller size is better for Tri/TT bikes for a more aero position.

I have a Slice 4, it's a 56cm, and I'm right at 5'11", fits perfect. Your mileage may vary based on your body proportions though, torso length, arm length, etc. I actually have my Slice for sale, too, not using it anymore.

SOLD


Thank you.

+1. 5' 11" here too.
I had a 58 I got a great deal on, but it was too big. Sold it and got a used Cervelo 54. Had to put a longer stem on, but it otherwise fits perfect.

The geometry on triathlon bikes vary dramatically by brand. You have to really look at the sizing, and do not go by the size road bike you ride. I am about your height (just under 6') and would probably start with a 56cm Slice. If you can get a sense for your stack and reach that is the best place to start, and really the way to look at tri bike sizing.

Good luck!