Well to be precise they can very from 4 inch to 8 inch depending on what species of bristle nose you get. There are around 60 species in total. They do best kept at temperature around 23 - 26 degrees c. Excellent in community tanks as they are peaceful and minimum tank size is 20 gallons. L number will depend on what species you go for and a male will grow at a quicker rate than a female
My golden male bristlenose in about 41/2" but my female is only 2", i also have 6 L318s they only get to about 2 - 3".
Very peaceful fish also good in planted aquariums mine leave my plants alone.
There are lots of L numbers in LFS these days.
Hope that helps