It depends on the weight of the fabric. The fairly light weight white linen pocket square I have is 12" (30 cm), though I also offer them as 16" (41 cm) for the occasional customer that likes bigger squares. I've never found it to be too stiff; even when I first started making and wearing pocket squares. Certainly there are heavier weight linen that I do find to be too stiff and heavy. In the above video that pocketsquareguy posted, the guy says he likes 33 cm (13").