Looking at the similar geometry of my current hard tail, size-wise it sits somewhere in between the small and medium 5-spots. I currently use a 70mm stem with an effective top tube of 575 mm, or 645 in total. This roughly equates to the small with a 75 mm stem, or a medium with a 60 mm stem. Based on the numbers, I’m leaning more towards the medium but I suppose the proof is in the riding.