It is possible to attach/send 25 MB over all ticket attachments via ticket attachment option.
It is possible to attach/send 128 MB at once via chatattachment option. However it is very likely that yours or customer's email client won't be able to accept such a large attachment, in case you will be working with the forwarded message or chat transcript.
This size limit is related only to LiveAgent cloud-hosted (SAAS) accounts.
Self-hosted licenses of LiveAgent have 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.