MTB 29er sizing help!

It has been a while since I've been around! I'm getting into MTBing( to race) and am in the process of ordering a bike from my team store. The issue is they don't have one in stock for me to try..

Being that I am almost 5'6- anyone have any thoughts on going with a small or a medium on the Specialized Era(29er).

Thanks!

Long arms or short? I'm 5'4" but have long arms, and would probably lean towards the larger frame because of that. Otherwise, I feel cramped. My current MTBs have ETTs of ~58cm, give or take. One is labeled small and the other is labeled medium, but they both fit.