You can 'hack' it by inspecting the element, then changing the max length:

Or you could add a js file to the library with the following code, so it changes it for you each time:

$( document ).ready(function()
{
SetContentSize();
});

function SetContentSize()
{

var block = crm.getArg("blockname",crm.url());

if(block!="")
{

$("#Contents").attr('maxlength','1000');
}

}

The column in the database is named 'CObj_Properties' in the custom_screenobjects table. It is an NVARCHAR(MAX).