One of my silverwood trees has converted a 15x15 area. It has no node in it at all, which is confusing, as I thought only trees with nodes would convert the biome. I have another tree with no node that didn't convert. The other that didn't is a fairly small tree with no side branches, the one that did convert 15x15 does have branches. No idea if that is significant.
So, my observations so far are:
Small tree, no node, no conversion.
Large tree, no node, 15x15.
Large trees, with nodes, 17x17. Node types are unknown as trees have been cut down.