The width of a QSFP-DD (Quad Small Form-Factor Pluggable Double Density) module refers to its physical size and is defined by the Multi-Source Agreement (MSA) specifications. The QSFP-DD module is an evolution of the original QSFP and is designed to accommodate higher data rates, including 200 Gigabit Ethernet (200GbE) and 400 Gigabit Ethernet (400GbE).

The QSFP-DD module has a larger form factor compared to the traditional QSFP module to support the additional electrical lanes required for higher data rates. The width of a QSFP-DD module is approximately 18.35 millimeters (mm), which is wider than the original QSFP's width of approximately 18.0 mm.

The increased width allows for the integration of more electrical lanes and optical fibers within the module, facilitating the higher data rates and capacity required for next-generation networking applications. It's important to note that the exact dimensions may slightly vary between different manufacturers, but the QSFP-DD standard ensures compatibility and interoperability among compliant devices.