I just got the pack of connectors and 2 lights from Menards and thought the price was quite reasonable for all the different bits. A few weeks ago, I dropped by the Electrical parts store and picked up some of the plugs you are asking about both male and female for extending and wiring in extra plugs and lighted accessories and found the parts more expensive then what I got in the pack from Menards. Sorry just can not find the package to tell you the size. You might take a plug or lead into an electrical store to match up what you need. There were 2 sizes that were very close so make sure you test it. Perhaps someone else has the correct measurements. I am sure you can find some parts on e-bay. If you are willing to solder the plugs will be much cheaper.
Thanks for the replies!
FWIW, I agree that the Menards power supply and connector prices, like most of their prices, IMHO are reasonable, but my connector needs were not precisely and efficiently met by their current offerings. Since component prices (especially when shopping overseas) are ridiculously cheap and since Radio Shack went virtual a few years back, we've not had a local electronics store to browse as you suggest, I decided to float my (virtual) request.
After posting I dug out an old metric ruler and made some measurements of one of the 'gen-you-wine' Menards connectors, then went on-line to see what was close. I quickly came to the same conclusion as PRRick (noting also that this type of connector comes in two common plug lengths, 9mm and 14mm, and Menards apparently uses the shorter 9mm length) and fired off a "slow boat" order for some cheap parts (several multi-Y connectors, and pre-wired male and female connectors), calculating that they were cheap and in any eventI could use the connectors for other projects if I was off-target, but it's great to get confirmation that I was on the right track after all! 
