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Node B will only receive messages that are published after it has connected (with the execption of latching).
There is a queue size for messages, that you can define, but it is not for a topic in general, but for the publisher/subscriber.
These things come clear, when you look at what a topic is. Messages are not sent to a physical topic (i.e. some program) and stored there, where subscribers receive them from. A topic is merely a common name to publish/subscribe to, but nothing more. Each message stream is handled out between the publisher and subscriber directly, they just use the topic name to "find each other".