How big a bump means something from skin test for tuberculosis?

Depends. If you are immune compromised or a child, a bump greater than 5mm in diameter is significant. Otherwise, a bump greater than 10mm in diameter is considered significant.
15mm or.... If the "bump" is bigger than 1.5cm (about 3/4 of an inch)it is considered positive. Also. If blisters are present, the test is considered positive.