Need help with my final project design (Ethernet Transmitter)

What interface are you planning on using? the packets will not arrive all in one go, they will arrive at a rate of 1 dword (being the width of the interface) per clock, (or multiple clocks)

Eg. for axi streaming you will receive 32 or 64 bits/ clock (depending on what interface size you chose). There are a few sideband signals to tell you things like whether the dword is valid or whether its the last dword in the packet. It is then up to you to decode the header.

You need to think like a hardware engineer. Your interfaces are always a fixed size - never dynamic.