Measuring power transformer capacity
About 50 years ago there was a rule of thumb that 1 sq.cm of main magnetic core can handle 10W of power. This was for 50 Hz AC. For 60 Hz it should be 20% better. The current capacity of a winding obviously depends of wire gauge, there could be low-current sections of the transformer, but it should be a good indication of carrying capacity (since no manufacturer would waste copper unnecessary). You should use transformer design rules when estimating current capability of a winding.