The maximum size for any single email message including attachments is 25 MB. The mail client (ex. Outlook, Apple Mail etc.) you use to send messages may also impose a limit on the size of any single attachment. For Outlook on the Web, the single attachment limit is 10 MB. For example, a message may have two 10 MB attachments (total = 20 MB), but not a single 15 MB attachment as that would exceed the single attachment limit. This is subject to change.
For the latest information, see this Microsoft article.