How do I avoid keeping copies of messages in Mail?

Question

I have a really low capacity MacBook Air. Earlier this year, I discovered I could keep it usable without a bunch of "running low on disk space" warnings by limiting the caching performed by Safari and Mail. In Safari it was easy, it's in Preferences, but in Mail it was a bit buried - select the account, advanced tab, and then "don't keep copies of any messages".
Problem is, it seems like in Mavericks this preference is gone. Is there an equivalent? I'm out of room on my Air once again.

Answer 1

@Lanny: For the record, I am not keeping a copy of every single email–emails are kept selectively and folders are well-organized. Attachments are stripped unless strictly needed. The problem isn't just size, it is the amount of files copied, and the number of devices onto which your emails are duplicated across. It is just wasted bandwidth and storage space. Nevertheless, even with selective mail handling the total amount of storage  easily number a couple of gigabytes if you have a couple of years worth of emails.
Your suggestion to use POP3 instead of IMAP would be more relevant if Mail.app had retained the 'Do not keep copies of any messages' setting when setting up your account as POP3 instead of IMAP. However, it does not. The setting is not present anymore in the Mail.app version shipped with OSX Mavericks–regardless of whether POP3 or IMAP is being used.
Furthermore, what you describe as the 'main features and advantages of using IMAP' is simply not correct. Every mail client by default copies your messages from the server for use locally. Where IMAP differs is that one can subscribe to commonly-used folders (which will be synchronized and kept up-to-date on the client), whereas less frequently-used folders only reside on the server. These emails can be accessed quickly thanks to IMAP's efficient search mechanisms–both for header or full-text searches. So in short it has much better ways of locating your messages that conventional POP3 clients have. There is rarely a need to have a local copy at all.
It actually appears that you have mixed up IMAP and POP3, as the IMAP 'advantage' you are stating is specific to POP3. With POP3, email is  stored on the server until you check your email, where it is fetched from the mail server and stored locally–often with the remote copy expunged–which is the complete opposite of what is intended.
The differences and benefits of using IMAP vs. POP3 are explained here:
http://mail2web.com/blog/2010/02/pop-imap-difference/
Your other suggestion regarding upgrading the harddrive simply to store multiple copies of email is a rather questionable solution if you ask me, as the SSDs are welded onto the MacBooks and cannot be replaced without tearing it apart (or spending a huge amount of money to have it serviced by Apple in order to do so). There is also the question of justifying a purchase of a bigger drive just because Apple removed an option in one of their core applications.
So my question as to why this core IMAP functionality was removed still stands. This is clearly an impairment of functionality which should be dealt with by Apple instead of forcing its users to abandon the way mail is being accessed, or resorting to expensive and cumbersome workarounds.