Can I now assign a fix larger size via the dialog you will find attached?
If the wizard allows you to resize (grow) the virtual disk size, you should be able to do this.
Can I also deactivate the checkbox "In Dateien mit je 2 GB unterteilen" without data loss?
Not sure if this is possible for an existing virtual disk. Anyway, I would not recommend you do this. The split format makes it easier and faster to e.g. grow a virtual disk. With a monolithic format (a single .vmdk file) the whole file needs to be copied in order to resize it, whereas with the split format only the required slices have to be added.
(free English translation: "divide into files with 2 GB")
Übersetzungen werden meist überbewertet. 
André