It is possible to attach/send 128 MB at once via chat/ticket attachment option. However it is very likely that yours or customer's email client won't be able to accept such a large attachment. This size limit is related to SAAS (hosted with us) versions of LiveAgent only.
Self-hosted editions of LiveAgent has an option to modify attachment size since it depends only on the settings of web-server that hosts the LiveAgent installation. E.g. if you use apache webserver, then you can set both 'upload_max_filesize' and 'post_max_size' to the desired file size limit that is allowed to be uploaded/sent.
