What's the field type of the MySQL field? If it's text then you can put up to 65535 characters into it without a problem.
If the string is short enough, it may be that your using an illegal character like a ' or /. These need to be escaped by adding an addslashes() to the var that you're submitting.
Request-URI Too Large
The requested URL's length exceeds the capacity limit for this server.
request failed: URI too long
But as I mentioned in my previos msg - from PHPMySQL it works fine.
Many thanks, Pavel
And GET has probably higher priority the POST
The HTML specs [w3.org] define GET as the default method [w3.org] to submit forms.