HTML Email won't save; related to size
body_text and body_html are the fields in the database that store these values, and the are of type 'longtext', which are of size 4Gb. So you're probably not hitting this limitation.

More likely, it's your php.ini configuration that is limiting the size of your "posting" (i.e. post_max_size), or a mysql limitation (e.g. max_allowed_packet).

It's also easy to put way too much bad html into these fields, particularly if you're copy/pasting from say, Word. If you take a look at your source, you might find a bunch of html you can and should remove.