Attachments plugin by itself does not restrict the file size, but the IIS server does. Its default max file size is 4 MB if you want to increase it you will have to go to the primary web config and set this two settings in it.
Note that if you have those items (system.web, system.webServer) already defined (by default you do) you will have to set the security and httpRuntime (by default you have httpRuntime but without maxRequestLength) in them. If you dublicate them it will throw an error which tells you that your configuration is invalid.