Netrounds Test Agents use the CUBIC TCP implementation. CUBIC uses an optimized congestion control algorithm for high-bandwidth and high-latency networks.
See http://en.wikipedia.org/wiki/CUBIC_TCP and http://tools.ietf.org/html/draft-rhee-tcp-cubic-00 for more information on CUBIC.
Netrounds does not tweak the Linux default settings in any major way, as the PC would then no longer behave as a normal user PC – which it should, since quite commonly a measurement is made precisely in order to detect or prevent possible user experience problems.
To select the TCP window size, Netrounds uses CUBIC's window scaling algorithm. The window size therefore varies, but Netrounds does have default and maximum window sizes predefined.
Packet sizes are again controlled by CUBIC. Normally, the packets will have full size (around 1500 bytes).
For Netrounds Embedded Agents, the TCP implementation and settings depend on the operating system of the platform used.
Note, finally, that when validating performance it is often useful to complement TCP testing with UDP measurements.