How much data can be sent via $_GET

Although the specification of the HTTP protocol does not specify any maximum length, practical limits are imposed by web browser and server software.

There is no limit defined in the RFC, but browsers limit the URL length (including get variables). IE for instance limits the URL length to 2083 characters, Opera about 4,050, Netscape 6 about 2,000 characters. A general rule of thumb is, that you shouldnt use more than 256 characters.

There is not only the PHP limitation, but you should also consider 'in between' proxies and the client software.

The http standard doesn't pose a limitation, though.

(I got this from here, where advice is to not exceed 255 char's urls!)