Neville: I researched it for you. Yalkut Yosef Chelek 1, Siman 19, Se’if 2. Loosely translated:
Many have the Minhag to not make a Brocha on a Tallis Koton at all for several reasons; and they have it in mind when they make the Brocha on the Tallis Gadol.
There are several reasons:
– Maybe isn’t large enough – doesn’t have Shiur because it shrunk in the wash.
– Maybe one doesn’t make a Brocha on a garment not worn in public (it’s worn under clothes)
– They don’t want to enter a Machlokes regarding which Brocha to make on it.
– They don’t want to enter a Machlokes if the walk from home to shul is a “Hefsek”
However, tehnically, one is allowed to make a Brocha on it, if it the correct size and the tzitzis didn’t rip….