From some notes I found somewhere.. they may be outdated.
While an IP packet can be as large as 65,535 bytes, packets are fragmented so as to fit into the path MTU. MTU = "maximum transmission unit". The path MTU is simply the lowest of all of the MTUs on all of the links between the end points. Each node along the way informs the next of it's MTU in the header.
Most/all IP packets will encounter an Ethernet link somewhere along the way, if nowhere else at the user's premises. Therefore, for all practical purposes, the maximum size of an IP packet (or, technically, fragment) on the Internet is 1500 bytes.
Subtracting the header, that leaves 1480 bytes for data payload.
What I gathered from that is that an Ethernet link becomes the bottle neck for all practical purposes. BUT if you are testing within a network then you mayhave different situations in which you have no bottleneck, or a larger bottleneck. Just a possibility.