Thanks @UnrealEverything. Crazy structure. Made a struct with NetSerializer and replicated it. Placed a breakpoint in NetSerialize and the Debugger showed me that Unreal uses FNetBitWriter (which is a child of FBitWriter) for net serialization. Had a go with that.
bool returnValue;
TArray<uint8> binary;
FMemoryWriter writer = FMemoryWriter(binary);
FVector vector(100.f);
writer << vector;
UE_LOG(LogTemp, Error, TEXT("SizeOfVector: %d Bytes"), binary.Num()); // 12 Bytes
FNetBitWriter writer2 = FNetBitWriter(1000000); // init with max bits
FVector_NetQuantize vector100 = FVector_NetQuantize(FVector(100.f));
vector100.NetSerialize(writer2, nullptr, returnValue);
UE_LOG(LogTemp, Error, TEXT("SizeOfNetQuantize100: %d bits"), writer2.GetNumBits()); // 28 Bits == 3,5 Bytes
Pretty good results. Thanks for your help guys.
Edit: Noticed I used FVector_NetQuantize in this test, NOT FVector_NetQuantize100.