Do you have any spare disks in the aggregate?
If so, the failed disk would have copied its data over to that one and the disk you are replacing will become the new spare.
You can replace the spare 300 GB with a 144 GB disk.
If you have no spare disks, then pulling the 300 GB disk and replacing it with a 144 GB will trigger a rebuild, but should be ok as long as only 144 GB worth of that disk was in use.
You can verify which disks are being used in the aggregate by running "aggr status -r".