This pages illustrates what you can expect when a limit on the maximum size of attachments has been configured for a project.
Assuming the following setting:
When an user try to attach, to an issue that belongs to the project Sample Project, a file exceeding the max allowed size, an alert appears and the file is not attached to the issue.
On the issue Create and Edit screens the alert displays as follows:
On the issue View screen the alert displays as follows: