Unfortunately, in Platform 4.2, is not yet possible to export xlsx excel files, and as far as I can know it won't be available in the 5.0 version.
By the way, why do you need and/or want to export xlsx excel files? One of the advantages of exporting in the old xls format is that all MS Excel versions open this format.
The fact is, for most people using the platform, exporting such large data sets to an excel is not a common scenario. If the platform were to support both xls and xlsx that would add additional complexity for the developers designing the applications. Only supporting xlsx is not an option as a lot of people have not upgraded to Office 2007 yet.
Probably the best solution for your scenario is exporting your data set to a CSV file using a custom component such as this one:
Hope this helps.
All the best,
Indeed you can open xlsx files in an early version of the MS Excel with the converter you are referring.
The problem that Hugo is facing is the large row/column number, and this converter do not resolve this problem. As you can see in ‘Office Excel 2007 features that are not supported in earlier versions of Excel’ page you need to cut the contents outside the MS Excel 2003 limits (256 columns by 65,536 rows) before converting the file.
So, as Pedro Oliveira indicated a good work around is to export your data (with large number of rows or columns) to a CSV format file. If you're having problems with quotes, commas or other characters, MS Excel also opens other type of files, for instance html files, that override this particular problem.