Packet sizes vary greatly, depending on the type of TCP/IP traffic, not much at all on the operating system. I think you are probably confusing this with 'window size'. NT uses an 8,760 byte window size on Ethernet networks.
Other relevant figures include the MSS (Maximum Segment Size) and MTU (Maximum Transmission Unit). To explain how the window size is calculated based on these figures would be complex and boring, so I won't detail it unless you ask.
MSS is calculated by subtracting 40 bytes from your MTU. MTU typically depends on the type of network you are connected to, but it's commonly 1500 bytes. That might be the number you are looking for.
To learn more about TCP/IP parameters that are stored in the registry, look at 'regentry.hlp' which is a help file in the NT Resource Kit.
Why are you curious about packet size?
ShackDaddy