its pretty impossible to put a number on it. Mainly a "large" site is going to be one that eats up 10% or more of the available shared server resources and is finding itself getting certain scripts disabled or perhaps in rare cases suspended for repeat problems.

So you could use a lot of transfer and have lots of hists and if your site is static use hardly any resources. If you have a PHP / MySQL driven website like most of us, then the same figures for hits and transfer can vary greatly based on how efficient your PHP scripts have been coded/optimized.

For example if you are running a PHP nuke site where the code is in general, extremely inefficient you might use the same resources as a site getting 5-10 times the traffic and hits on a more professional script. for example, these vBulletin forums for the most part run very efficiently in comparison to some of their open source counterparts.

A lot of "large site" can also mean userbase. Dedicated servers are notably 'up' a lot more than shared servers and once you reach a threshold of subscribers or visitors those 99% vs 99.5% vs 99.9% is going to become important whereas a less busy site might not even notice 97% uptimes.

So the question is hard to answer, it is similar to the question;
"How long is a piece of string?"

Generally, if you haven't already made the move, your host will suggest that you are ready for a dedicated server, or *some* (quite a few these days actually) hosts will suspend your site until you agree to buying a VPS or dedicated.