Is there any way to make icon size on task list bigger, so that it could be almost as big as the size of a shortcut/launching icon on the panel? I mean as big as task list's height because task list's height is a little smaller than panel size.

I don't think it's possible with the default task manager, but you can configure it in e.g. Smooth Tasks. Not sure if there's any easy way to install it in Kubuntu, as the original version is outdated and (as far as I know) won't compile using newer versions of KDE Platform. Here's a fork that I believe works: https://bitbucket.org/flupp/smooth-tasks-fork

re: smooth tasks - there is Smooth Tasks 2 that's supposed to work with 4.8 http://kde-look.org/content/show.php/Sm ... ent=148813

update - I can confirm it works on 4.8.95

Thank you for response. I installed Smooth Tasks 2 but it was buggy. With smooth-tasks-fork I had compilation errors.
I decided to fill in a wishlist bug report, because I would like to have bigger icons on default task list widget. I am not a big fan of different icon task widgets anyway.
Bug 303787 - task list icon size - make it bigger
https://bugs.kde.org/show_bug.cgi?id=303787

Edit:
I use smooth-tasks-fork now. It has a lot of option that allow to customize the look and behaviour, which is great. It is important to check out the correct branch. The command is:
hg update -r kde-4.8
Thank you for reference.