When I built my first Database Driven CMS many years back I encountered a similar behaviour. The problem may be the format of the "Content Area" in MySQL. It is likely set to TEXT, but if you are running out of room, you may need to set that to "MEDIUM TEXT or LONG TEXT".
If my memory is right, a TEXT column can hold 64K worth of data; LONGTEXT can hold 4GB. Not sure what the MEDIUMTEXT holds.
That would be a database answer - but you should wait for someone from WebAssist to respond, as PowerCMS may have it's own limitations.
FINAL NOTE: If you change any database settings be carefull. If you set it to the wrong value (ie: date, or int) your text could become truncacted and you will lose data. Backups are your friend.