eZStringType uses data_text1 which is limited to 50, while it should use data_text4 with a limit of 255.
ezstring default value should not be truncated
BTW: design\standard\templates\class\datatype\edit\ezstring.tpl should be changed as well with maxlength=255 for the input field.
In fact the data_text1 ... data_text4 fields in ezcontentclassattribute should all probably be of type Text, not varchar.
If you look at their definition in the php class, they are set there as Text...