ePubs are designed to adjust to the individual device and, in all honesty, do a better job than our previewer gives them credit for! We have plans to update the previewer in Atticus, but I would recommend proofing your ePub with Kindle Previewer - https://www.amazon.com/Kindle-Previewer/b?ie=UTF8&node=21381691011
If it looks great there, it will show up similarly in most devices. Of course, some users will have their font set to extra, extra large for visibility, or some other criteria like that, but for the most part, Kindle Previewer is the most accurate reflection of most devices.