Maximum Attachment size?
Not with Yosemite!
In Yosemite, that entire procedure is baked in to Mail—and it’s all stuff that happens behind the scenes. If you want to attach a large file to a message, just drag it in and send away. Mail will upload the file to a temporary holding bin on Apple’s servers, where it will remain for 30 days. Then the attachment is replaced in your email message with a download link to the file. (If your recipient is using Yosemite, Mail will just download the large file automatically, as if it had been attached to the message.)