Consider a not-styled graph:

g = RandomGraph[BarabasiAlbertGraphDistribution[100, 1]]

Change all vertex sizes relatively to plot diagonal

SetProperty[g, VertexSize -> {"Scaled", .02}]

Change single vertex color and size relatively to plot diagonal

SetProperty[g, {VertexSize -> {10 -> {"Scaled", .04}}, VertexStyle -> {10 -> Red}}]

Now consider styled graph

h = RandomGraph[BarabasiAlbertGraphDistribution[100, 1], GraphStyle -> "LargeNetwork"]

Change single vertex size relatively to plot diagonal

SetProperty[h, {VertexShapeFunction -> {10 -> "Circle"}, VertexSize -> {10 -> {"Scaled", .04}}}]

Use another method and add color

SetProperty[h, VertexStyle -> {10 -> {Red, PointSize[.04]}}]