I would make sure no one else is using the file. Word will give a similar error when someone else is using the file. Usually, it will offer to notify you when the file is available for editing or open read only.

Also, try a right-click on the icon in Windows and choose 'properties'. Make sure that the file is not 'read-only'. If it is, then uncheck the box.

There are so many possiblities with this type of error but I would start with this. If it does not help, then let me know and I can offer another solution. I am not aware of any file size restriction. Just to humour me, let me know what size the file is.