When I designed the "Officer" model sgian dubh I measured several dozen sgians that were manufactured between 1860 and 1958 (dates based on hallmarks). The variation in overall length was between 6 and 7.75 inches. Interestingly, blades were a more-or-less uniform 3.5 inches (give or take a quarter of an inch or so). Hilts showed the greatest variation with the shortest measuring a scant 2.85 inches, with the longest topping out at over five inches. Elaborate pommels and exotic antlers added to the length of some of the hilts.

Taking two factors in to consideration (aesthetics and balance) The following dimensions were settled upon:

O/A Length: 6 5/8 inches;
Blade: 3 1/2 inches;
Hilt: 3 1/8 inches;
Max. Thickness of Hilt not to exceed 3/8 inches.

In other words, this particular knife fell into the size range of about 90% of the sgians measured before I set about designing this particular sgian.

I think you are safe in considering anything UNDER 7 inches O/A length as suitable for use as a sgain dubh. Once you go over that, it generally becomes too heavy and too unwieldy to be worn tucked into the top of your hose.