SQL server 2012 AG adding DR Node

Hi,

I have setup SQL Server 2012 AG with 2 node. Mirroring is set as Synchronous Commit. I have used 1 TB disc on both server. Now i am going to add 3rd node as DR node.

For 3rd DR Node i have 500 GB Space. Please will it be ok to install and add 3rd node with less space. OR Space should be same as other node.
Right now i m using 200 GB and growth is not such.

Secondly when i add 3rd DR node how and where i specify mirroring as Asynchronous.

Please guide.

Thanks

iffi