The DAG started at 1 GB at the time of the Frontier launch, and increases by approximately 0.73x per year. That puts the current size at roughly 1.35 GB as of mid-January 2016 (feel free to comment with an exact link if anyone has one).
Following the same crude approximation:
- The 2GB limit will be hit around mid-December 2016
- The 3GB limit will be hit around mid-April 2018
- The 4GB limit will be hit around mid-September 2019
However, it's likely that the switch to PoS will occur long before then.
For what it's worth, certain GPUs require the DAG to be uploaded in chunks. You should search on the forums with the name of your card to find if this solution has worked for other users running the same hardware.