I found this solution in the Spiceworks VMWare Forum by User ZEFiR and after testing it succesfully with ESXi 5.5 Update 2, I wanted to preserve it for future use. The Original Post can be found
https://community.spiceworks.com/topic/411970-installing-esxi-5-5-with-4gb-ram-memory_size-error?page=1#entry-3816226. Thanks a lot to ZEFiR for sharing this very useful piece of knowledge :)!

1. Boot from VMware ESXi 5.5

2. wait “Welcome to theVMware ESXi 5.5.0 Installation” screen

3. Press Alt+F1 for enter console and login as “root” (no password needed)

4. go to:

cd /usr/lib/vmware/weasel/util/  

delete file:

rm upgrade_precheck.pyc;

5. remove readonly flags:

mv upgrade_precheck.py upgrade_precheck.py.def  
cp upgrade_precheck.py.def upgrade_precheck.py

6. set permissions:

chmod 755 upgrade_precheck.py

7. open script in vi:

vi upgrade_precheck.py

8. find “MEM_MIN_SIZE”,

pres "a" (edit mode. "(4 * 1024 - 32)" change to "(2 * 1024 - 32)", pres 'ESC', run command ":wq" (save & exit)

9. look installer PID:

ps -c | grep weasel

10. kill installer (PID = process Nr.):

kill PID

Installer process run automatically (with modified config script).

Continue from “Welcome to theVMware ESXi 5.5.0 Installation”