The pattern for all four of my Celtic crosses is now on sale on Ravelry – http://www.ravelry.com/patterns/library/celtic-cross-bookmark
My original cross.When I used #10 crochet cotton and a 1.25mm hook (Blue cross) it came out about 4 inches high and three inches wide. (10 x 7.5 cm) However using a smaller hook will give a firmer though slightly smaller bookmark. (Red cross)You could also try multi-coloured thread. For this I used a 1.25mm for the starting chain and a 1mm hook for the rest. I also increased the picot stitches from two to three.