300 Tdi glow plug connection: what size is the nut on the electrical connection?

My Defender: 1996 300Tdi 90 CSW
I am about to replace the glow plugs, the old ones appearing to be u/s.

I have purchased 4 x Bosch ETC8847G. Strangely, they do not come with new nuts/washers on the connection stud.
The nuts on the old ones were steel, but were quite rusted due to age. Given that their only function is to secure a good electrical connection, it seems like it would be better to fit brass ones.
Can anyone tell me what size these nuts are, or if there is a better solution?
Thanks.

Metric… think 4 or 5 mm

97 D1 300 tdi - 7 mm socket fits perfectly

my nuts are all silvery and have never replaced the glow plugs

M4 for a 7mm spanner (usually)

Thank you all for your responses.
I don't have a pair of accurate calipers, but checking with a rule this morning it looks like 4mm. So, will obtain some new M4 nuts and washers.

Separately, I have never checked my glow plugs before (1996 300 Tdi, nearly 5 years in my ownership) and I have only had one instance of difficult starting which was on a very cold morning last November. So, I suspect that many owners have failed glow plugs, but don't know it.