I don't know where to start with this one really and I can't upload an LSA because of the data in it but here goes anyway:

I have a VERY large survey (with timings, tokens and questions I have 785 columns in the export and 479 result rows!) and my LimeSurvey instance is sat on SQL Server Express 2012. Exporting results now fails and here's why:

\application\helpers\admin\exportresults_helper.php function loadSurveyResults approx line 320 executes the YII query builder like this:

$survey->responses=$oRecordSet->select($aSelectFields)->order('{{survey_' . $survey->id . '}}.id')->limit($iLimit, $iOffset)->query()->readAll();

Using this before this line shows the query looks like the example below:
echo $oRecordSet->getText();
die;

SELECT *
FROM [survey_816525]
LEFT JOIN [tokens_816525] [tokentable] ON tokentable.token=survey_816525.token
LEFT JOIN [survey_816525_timings] [survey_timings] ON survey_816525.id = survey_timings.id

When executed in SQL Express Mgmt Studio this returns error:

Msg 511, Level 16, State 1, Line 1
Cannot create a row of size 12454 which is greater than the allowable maximum row size of 8060.

Which it seems to because of SQL Server's 8k limit on the 'work table' used to build the query results: http://msdn.microsoft.com/en-us/library/ms186981

Interestingly I can get the results to return if I remove the order method in the YII call like this:

$survey->responses=$oRecordSet->select($aSelectFields)->limit($iLimit, $iOffset)->query()->readAll();

but of course the results are returned in no particular order.