Size is not everything!
First and foremost a serama should have good type. That means a proud pouty chest, a high tail, a very short V back and when alert to attention wings that point near vertical downwards. Also bare in mind the smallest birds are often very unreliable as breeding birds. Hens under 250g often do not produce viable eggs and cock birds likewise are not as reliable fertility wise when under 300g.
Size B birds often make the best breeding birds producing sizes A, b and the odd C. (some A birds however can produce size C birds, and size C birds can sometimes produce a size A bird).
Having said all that the size A birds are what we all tend to prefer! They are a very tiny breed and have a real wow factor in the smaller sizes.
Serama are not judged in class weight bands however it is traditional in Malaysia to call certain weights certain classes and it has caught on here as an easy shorthand to tell a smaller bird from a larger bird.
The weight classes are as below:
Mature Serama Males
Here is a class A hen
Here is a class B nearly C hen