I have just received a roll of 5mm galvanised wire from jacksons fencing, and have made some hoops – works fine, bit tough to bend – a bit of elbow grease is required, and you would need some cutters that can handle 5mm wire.
5mm seems similar to Charles’s hoops, which I have seen – they are definitely not 4mm which I found last year to buckle under wind and rain. I would definitely recommend trying the 5mm.
If you are using 4mm it would be worth 2 hoops to a metre.
6mm wire I couldn’t find, I don’t think anyone makes it anymore, I found a 2nd hand roll on ebay in Scotland but it weighed 190kg! 
Happy cloching!