The truth is, size matters! At least when it comes to pocket squares. A pocket square of premium quality made of silk should be around 40cm in order for it to stand up in the pocket. Smaller versions have a tendency to fall down in the breast pocket, becoming barely visible to the viewers eye. You should however look for a slightly smaller handkerchief if you select a more stiff material like linen. Due to their structure linen will stand up more in the pocket and therefore risks to become to visible and take over the entire outfit. For linen a better size would be around 35cm.
A premium handkerchief should have hand rolled edges that gives them a nice touch and shows that a lot of care has gone in to that particular product. Also look for a good print. Handprinted squares has much more richness and depth to their colors opposed to those with digital printing.