First, let me just say, Hank, if it ain't broke, don't fix it, and second, no, he's not right, unless you're growing mini cucumbers - rhs page on cucumber growing in pots says 10 inch pots.

https://www.rhs.org.uk/advice/grow-your-own/vegetables/cucumbers

Thanks very much B, I thought I was on the right track but "know-all" ( as he is known) put doubt in my mind.

Sounds like one of my sisters - always has lots of 'information' to pass on, but its almost always wrong.. thank heavens for the internet, I just check whatever she says online.