Hi David,
There is no specific limitation for the file size. It depends on the results stored in the file and the amount of available memory. We will think about starting to store results in individual files in one of future updates to avoid problems with high memory consumption.
Please note that, using the current version of AQtime, you already can store results in individual files. To save results to a file, right-click the results set in the Report panel and select the 'Save to File...' item from the context menu. To load results from a file, select the 'Load From File...' menu item.