I will be starting a deployment and find that they currently have 600+ phones and 1500 DID’s.
We are use to doing sites with up to 300 phones and 500 DID’s…
Will FreePBX handle a site this large? What kind of server hardware should I look for if it can?
Thanks
There are no hard rules, an old rule of thumb in the old telephony world is you provide 10% total capacity. (That breaks if you are running a call-center and here in California after an earthquake 
. )
It all depends on your transcoding/recording needs but just google for “asterisk dimensioning” for many views.
I personally would install sysstat on one of your extant deployments and then you can analyse a week later with sar, scale to suit . . .