There are a couple of ways to deal with this issue. Surprisingly, the most low-tech solution is what I've found to be the most effective.
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Let's say your ISP has a cap of five megabytes on email message size. You start sending your 24.5 megabyte behemoth, and five megabytes in your ISP's mailer says "Nope, too big - FAIL".Outlook doesn't realize that that's not going to get fixed and treats it like any transient error that might not happen if it tries again. So it tries again. And again. And again.
With no actual hope of success.
Meanwhile, once you realize that there's a problem, you can't do anything about it, because Outlook has the message while it's trying to send it. You can't delete or move an email that Outlook is in the process of sending.
There are two ways I've approached this problem in the past:
Get lucky - there's a small window of time between the failure and Outlook's next attempt to send it when it does not have the message locked. If you're lucky you can delete it during this time. But you must be really lucky.
"Outlook doesn't realize that that's not going to get fixed, and treats it like any transient error that might not happen if it tries again. So it tries again."
Pull the plug - not the power, but your network. Literally disconnect yourself from your network. Outlook will not be able to contact your ISPs server at all and won't even begin to try sending your message. It might take Outlook a few seconds to realize that it's off line, but once it does, you can delete or move the message. After that, reconnect to your network and get on with your 
