Pretty much the best work I've ever see in this technique is done by Beryl Dean, who studied the Apprenticeship at RSN years ago (if anyone went to the 2011 Knitting and Stitching Show, there were some utterly fabulous examples of her work there.)
But back to me and my tired eyes..... these following pictures were really just personal experimentation with keeping my stitches even, how intricate I could make the lettering, and how small different fonts could go without losing the detail: (yes, I work pretty tiny)
Gold: 4cm square
Red / black: 3cm square
Plain black: 2.5cm square
And then I just couldn't leave the frame alone.....
6.5x 2.5cm -- the text came from a flyer I had lying around that read 'South Coast Tattoo'. I didn't think it was possible to get this small detail, but hey, it seemed to go okay. The way to do it is complete each horizontal line at a time, working along with a needle threaded up in each of red and black, and completing all the stitches in that row. Then you move up or down one, lay another silver string across, and carry on putting the couching stitches in place. It's got much more sparkle than this and really catches the light - you could use any couching thread you like, I guess, but this is a neat way of incorporating metallics in without having to thread them up in your needle (which is a pain at the best of times.)