Hello anzrysullivan,

The maximum filesize depends on your server settings. There is no limit in the Content Uploader. Typically between 1000 and 2000 articles shouldn't be a problem.
xlsx files shouldn't be a problem as well. Maybe there is something wrong with the file. If you like you can send me one of the xlsx files which doesn't work and I;ll try to figure out what's causing the problems. If you have the choice you should save the file in Excel 97 format (XLS) .

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Kind regards,
Pete

There is nothing wrong about using xlsx files. Basically all spreadsheet filetypes are supported (csv, xls, xlsx and ods), however if you're experiencing problems with a certain filetype it makes sense to switch to xls as this is the one that has proven to perform best.

The server settings that influence the number of possible articles uploaded at once are:
max_execution_time
upload_max_filesize
post_max_size

Kind regards,
Pete