I can't remember exactly but its big (for a drain plug) somewhere around 17 or 19mm.
15 mm
Need to know the year, make, model and engine size to help you with that one.
You need a 14MM Allen key or similar sump plug key.
The size wrench to remove it should be 15mm.
It seems to me I use a 15 mm wrench to remove the engine oil drain plug.
Every car and truck is different however most ford vehicles have a 13mm or a 15mm drain plug. I would use a wrench to remove and install the drain plug since it is easy to damage the pan if it is overtightened. If the plug has been replaced, it may have a different size.
13mm
My 92 has a 9/16 size plug My 1997 LeSabre has a 15mm drain plug.
15mm
15mm.
14mm
11/16th