The spell Negative Energy Flood (XGtE pg. 163) states:
A target killed by this damage rises up as a zombie at the start of your next turn.
Statistics for the zombie are in the Monster Manual.
This seems to imply that the creature killed rises as a zombie with exactly the statistics stated in the Monster Manual.
If I kill a gargantuan (or anything larger than medium) creature with Negative Energy Flood, does it rise as a size medium zombie?