It depends a lot on the species.
Species such as Hypancistrus sp. (Zebras (L046), Queen Arabesques (L260) etc.), usually max out at around 5 or 6 inches. Depending on the L no. Gold Nuggets (L018, L081, L085 and L177), which are Baryancistrus sp., are said to grow from a max of 7" (L081) to some that get over 14" (L018, L085).
I'm not sure of the max size Blue Spot, but from what I've read species of Spectracanthicus stay relativly small.
There are a lot of diffrent species of 'fancy pleco's' out there and their sizes can range from tiny, less than 4cm max (Otocinclus sp.) to over a meter (Acanthicus hystrix, Acanthicus adonis).
Cheers, CatBrat.