It's a very tricky installation if you want to create a smaller disk than the default one.
I have to disagree with that statement! IMO many Users just are not paying attention and or just outright do not read the product documentation before using it much less when they run into issues.
Case in point... One is at the Finish dialog box and as a example it plainly and succinctly says "To change the default virtual machine settings, click Customize Settings. To start the installation of Microsoft Windows now, click Finish." or translated (Um die Standardeinstellungen der virtuellen Maschine zu ändern, klicken Sie auf "Anpassen Einstellungen". Wenn Sie die Installation von Microsoft Windows jetzt starten möchten, klicken Sie auf "Fertig stellen".) so what is not understandable about a blatantly obvious statement and a Customize Settings button, the largest button there, that is centered in the lower portion of that window above the other control buttons? 
So I think it's quite fair to say a User was just not paying attention to what he/she was doing in situations like this! 
It takes but a few extra clicks and key strokes and should take less then a minute to make some customizations. Really absolutely nothing tricky whatsoever about it!
I'd just like to know if I understand the whole procedure correctly.If, for example, the whole situation is like this:
free disk space on Macintosh HD: 200GB
used disk space
on WinXP virtual disk: 10GB (of 20GB)
on Win7 virtual disk: 20GB (of 30GB)
Then I will be able to use 170GB on MacintoshHD, right? Even though the virtual disks are 20GB and 30GB 'big'.
If so, will I be able to delete files from the virtual disks and use the gained space in Mac OS? Let's say my virtual disk of 20GB is full and I delete 10GB of data, will there be 10GB more space on the MacintoshHD?
Hopefully I understand this now
Because if I do, it's not too complicated at all.
Because if I do, it's not too complicated at all.
Yes, of course you can use whatever free space exists on the Host's Filesystem however keep in mind that the amount of space the files that comprise the virtual hard disk will continue to grow as necessary thus reducing the amount of free space on the Host's Filesystem. So you need to be cognizant of this, that the virtual disk can/will grow in size up to the defined size and also that as you use the Snapshot feature the Virtual Machine Package itself can grow to many times larger then the defined size of the virtual hard disk.
As you delete objects from with the Virtual Machine Guest OS Filesystem the space taken up on the Host's Filesystem is not automatically given back and you'll need to preform routine maintenance from time to time. Have a look at: 
Clean Up a Virtual Hard Disk
The only thing still left unclear is that I really don't know what happened to the 40GB missing. After reinstalling WinXP with a 20GB virtual hard disk I did not lose any further disk space except the place the virtual machine itself uses. Even after I tried deleting it again and installing it for the 3rd time, there was no further Macintosh HD space missing. I guess these 40GB must have been used by something else.
Sometimes looking a the contents of the Macintosh HD from a graphical perspective can help understand what may be taking up space appropriately or something unwanted or no longer needed etc. and to that end using a program like like Disk Inventory X or GrandPerspective can help put things in proper perspective and help to remove the unwanted or no longer needed content. On a cautionary note though, do not delete anything that your not sure of that your deleting!
EDIT:I also got one more question there: When it comes to RAM, I can change the amount after installing the guest OS, right? Let's say WinXP may use 512MB and once in a while when I need more, will I be able to increase the amount of RAM WinXP may use?
Yes while the Virtual Machine's Guest OS is shutdown, not suspended, the amount of RAM may be adjusted up/down as one desires.