Hi Chris - are you asking what gauge wire in order to make your own (in which case WHS above) or to order from eBay on upholstery spring link such as this ....
http://www.ebay.co.uk/itm/Upholstery-Springs-Zig-Zag-Double-Cone-Super-Loop-Springs-FREE-POSTAGE-/151191722564?var=&hash=item2333ba9a44:m:mzOL9pjn7pmKVkgqL4TA9FA" onclick="window.open(this.href);return false;
http://www.upholsterywarehouse.co.uk/acatalog/Double_Cone_Spring.html" onclick="window.open(this.href);return false;
but if your measure your own .... this cut and paste might help
9 gauge (3.65mm) are used for chair and sofa seats
10 gauge (3.25mm) are used for dining seats, small chair seats and sofa backs.
11 gauge - somewhere inbetween
12 gauge (2.64mm) the thinnest are used for sofa and chair arms.
Cheers