Size of nut in the rubber well nut?

[amagalla]

I'm looking for the size of the nut inside the rubber well nuts (part number 999-500-092-00) that hold the 2 side plastic trim pieces near the firewall under the hood. There are 2 of them (one for each side). I know that I can buy them online, but was hoping to get them faster/cheaper. If someone knows, please let me know.

Thanks.

Tony