In my experience, the key concern about attachments is not the amount of space they take on the recipient's computer (most people have big hard drives these days), but whether they will cause your message to fail to be delivered. There are two issues that I frequently encounter in which attachments cause problems with email:
- Most mail servers have an upper limit on the attachment size they will allow. At the moment, this is often seems to be no more than 10MB, but I've encountered as low as 2MB.
- Aggressive protective software sometimes gets upset about attachments, especially known malware vectors like zip files.
If you trigger a filter like this, your email will disappear into a black hole and, depending on the configuration of the system that ate your email, you may not even know that this has happened. Thus, for any attachment above about 1MB or of an unusual type, I recommend sending a link instead.
Note, however, that links can also be viewed as suspicious, by either software or people, since there's a lot of nasty phishing schemes out there. Thus, I recommend not using link shorteners and if possible hosting an academic institutional server or on a well-known and semi-trusted service like DropBox.