Using trigger we can impose attachment Size in Salesforce before attaching it. Check the following trigger for reference.

Sample Trigger:

trigger AttachmentBIUTrigger on Attachment (before insert, before update) {

    for(Attachment att : trigger.new) {

        if(att.Body.size() > 3145728) {

            att.addError(‘Size cannot be greater than 3 MB’);

        }

    }

}