What kind of read and write speeds can I expect from this NAS setup with the 128TB configuration?

You can expect up to 2,315 MB/s read and 1,147 MB/s write performance from this NAS setup with the 128TB configuration.

What are the dimensions of the Synology RS1221+ RackStation, and will it fit in a standard 19-inch rack?

The dimensions of the Synology RS1221+ RackStation are 3.46" x 18.98" x 12.07" (with server handles).

double click image to zoom in

Synology RS1221+ RackStation with 32GB RAM and 128TB (8 x 16TB) of Synology Enterprise Drives Fully Assembled and Tested By CustomTechSales