How to fing the sises of selected set of directories
 

Is there a command to find the sises of the directories. (only a selected set of directories)

In a terminal window change to the directory you are interested in.
E.g.

cd myData

then type

du -h

to see documentation for du (disk use) type:
man du

If you want to see free space use df -h (disk free)

In KDE Konqueror you can right click on a directory and then select Properties in the pop-up menu.

It will show the size of the whole subtree.

Problem is that when du -h is used entire list of directories keep appearing. how to filter that

Use "du" in the directory above the one you are interested in, and use the -s (summary) option so that the contents aren't included. The manpage and "du --help" would have helped you find the answer.

try to grep the directory you want as 'du -h |grep direname'