There isn't an easy way of doing that right now. Given that a number of people have asked for it we're looking at adding it in future versions.
If you want to do this now, the only way to do it is by mucking around with the installer internals (and knowing how to use vi). It's not that difficult if you're familiar with using a command line. Try these steps for changing it with a graphical installation:
boot the ESX installation DVD
switch to the shell (Ctrl-Alt-F2)
ps | grep Xorg
kill the PID which comes up with something like "Xorg -br -logfile ...". On my system this comes up as PID 590, so "kill 590"
cd /usr/lib/vmware/weasel
vi fsset.py
scroll down to the part which says "class vmfs3FileSystem(FileSystemType):"
edit the "blockSizeMB" parameter to the block size that you want. it will currently be set to '1'. the only values that will probably work are 1, 2, 4, and 8.
save and exit the file
cd /
/bin/weasel
After that, run through the installer as you normally would. To check that it worked, after the installer has completed you can go back to a different terminal (try Ctl-Alt-F3 since weasel is now running on tty2) and look through /var/log/weasel.log for the vmfstools creation command.
Hope that helps.
--Patrick.
