Assuming that the audience was quiet, there's no reason that any of them would be unable to hear the actors.

Let's use the Globe Theatre (of Shakespeare fame) as a guide. It was a three-level open-topped structure, roughly 100' across, with a 45' foot wide by 30' deep stage. That's not particularly large as theatre spaces go.

An actor with any amount of training would have learned to project his voice. This is a technique that allows one's voice to fill a space without yelling or screaming, or to be heard over background noise or music.

Speaking personally, I come from a theater background, with a BFA in design and 2 years of MFA study as well. Even as a lighting designer, I learned how to project my voice. I feel confident that I could easily be heard clearly by someone in the third story of the Globe, as far away from the stage as possible. I understand this is anecdotal evidence, but I provide it as a touchstone nonetheless.

Now the question is if the audience would be quiet! There would be two "classes", if you will, of people attending a play at The Globe. The first would be those who could afford to sit in the "box seats" on the outskirts of the building... that person three stories up that I mentioned, for example. These people would be well-behaved, more-or-less respectable members of society.

But those who could only afford standing room only, and thus were in the "pit" directly in front of the stage, were called the groundlings. You know the old cliche of an audience throwing old fruit or tomatoes at a bad actor? Allegedly, the groundlings are the source of the trope. To say they were unruly would be something of an understatement; talking on a cellphone during a movie would be a vast improvement over their behavior.

So the actor of the time might have some trouble being heard over the "unwashed masses" of the groundlings, but otherwise, there wouldn't be much of a problem at all.

edit: spelling, grammar.

The Elizabethan voice is an extremely different concept for our own for various reasons. Patsy Rodenburg makes a convincing argument that everybody in that time period would have naturally had a much stronger voice, because people were walking over rough, unpaved terrain much more frequently than we do today, with considerably less support built into our shoes, etc.

The argument is basically that, from the ground up, people were more fit and actors even mores, given that they would have been traveling and performing throughout England. Owing to that, the Elizabethan voice would have had much stronger vocal and physical support, and, thus, would have had no trouble properly speaking clearly and consistently in a large, open theatrical space.