My smallest measure is "a gnats" (by inference - bollock).
i.e. "That needs a gnats more" (in order to fit).
A small piece of cake is "a slivver". e.g. when invited to have seconds, "I'll just have a slivver".
A second mug of tea is "A topper". e.g. "Would you like a topper?"
A tad more is more than a gnats.
A smidgeon is probably between the two.
