The digest version of the list is generated automatically by the list software. It is configured so as to make each digest message about 40K in size. This usually results in somewhere between six and ten digest messages per day, depending on list volume. Its size can not be set individually for each subscriber.
I get a trickle of emails asking for the digests to be bigger, smaller, more frequent, and less frequent, so I think I've found a happy medium.
Though the digest is a popular form in which to receive the Arlington list -- it is chosen by about half the subscribers -- it also has its drawbacks: it's harder to search and reply to, and it's impossible to sort individual messages.
You might want to consider switching from the digest mode to individual messages, and setting up a rule in your mail program to file those messages in a separate folder instead.