MaxSize is the maximum number of bytes you could have for the queue in memory.
PageSize, is the size of each file where you store the messages on Page.
Like, we could start paging at 100M, and each page (or file) will have 10M.
There is also the GlobalMaxSize that takes all the Queues in consideration. (we aways use the GlobalSize by default, and have the MaxSize on queues disabled).